Retina des Menschen und der Wirbelthiere. Zeitsch. f. wiss. Zool.
1857. Vol. VIII, p. 27.
*Müller, W.*, Ueber die Stammentwicklung des Sehorgans der
Wirbelthiere. Leipzig, 1874, Festgabe an Carl Ludwig; Abstract in
Centralbl. f. d. med. Wiss. 1877, pp. 372 and 388.
*Ogneff, J.*, Histogenese der Retina. Centralbl. f. d. med. Wiss.
1881, p. 641.
*Ogneff, J.*, Ueber die moleculäre Schicht und die sogenannte
reticuläre Substanz der Retina. Centralbl. f. d. med. Wiss. 1883,
p. 801.
*Oppenheimer, L. S.*, Die Stäbchen in der Netzhaut der
Froschembryonen. Schenk’s Embryol. Mitt. Wien. 1878, p. 163.
*Ranvier, L.*, Traité technique d’histologie, sixième fasicule. 1882.
*Ritter*, Ueber den Bau der Stäbchen und äusseren Endigungen der
Radialfasern an der Netzhaut des Frosches. Arch. f. Ophthalmol.
1859.
*Ritter*, Zur Histologie des Auges. Arch. f. Ophthalmol. 1868.
Vol. XI.
*Schifferdecker*, Studien zur vergleichenden Histologie der Retina.
Arch. f. mik. Anat. 1886. Vol. XXVIII, p. 305.
*Schultze, M.*, Zur Anatomie und Physiologie der Retina. Arch. f.
mik. Anat. 1866. Vol. II, pp. 175–286.
*Schultze, M.*, Ueber Stäbchen und Zapfen der Retina. Arch. f. mik.
Anat. 1867. Vol. III, p. 215.
*Schultze, M.*, Bemerkungen zu dem Aufsatze des Dr. W. Steinlin.
Arch. f. mik. Anat. 1868. Vol. IV, pp. 10–21.
*Schultze, M.*, Die Retina. Stricker’s Handbuch der Gewebelehre. 1871.
*Schultze, M.*, Ueber die Nervenendigung in der Netzhaut des Auges
bei Menschen und bei Thieren. Arch. f. mik. Anat. 1869. Vol. V,
p. 380.
*Schwalbe*, Lehrbuch der Anatomie der Sinnesorgane. Erlangen, 1885,
p. 392, etc.
*Steinlin, W.*, Zur Anatomie und Physiologie der Retina. Arch. f.
mik. Anat. 1868. Vol. IV, p. 10.
THE SKIN AND THE SENSE-ORGANS.
I. THE SKIN AND ITS APPENDAGES.
The general characters and the colouring of the skin have already
been described (pp. 4 to 7). The skin consists of the _cutis vera_ or
corium, and the cuticle or epidermis, and is possessed of numerous
glands.
*a.* The *epidermis* (Figs. 228, 230) consists of several layers
of epithelial cells, those of the deepest layer being more or less
columnar in form, those of the middle layer shorter and polygonal,
while those of the superficial layer are flattened, very transparent,
and horny.
All the surfaces of these cells are serrated, the serrations being,
however, with difficulty seen on the free surface of the superficial
cells (Leydig). Each cell has a distinct nucleus; in the deeper cells
the nucleus is oval, broad, and rounded, in the superficial cells
flattened and thin.
The surface of the epidermis (Fig. 230) forms a very beautiful
mosaic of flat cells, chiefly hexagonal in form, with pale, central,
oval nuclei. Here and there two adjacent cells appear to enclose a
semilunar space (Fig. 228 _D_); these spaces are occupied by peculiar
cells (goblet-cells, E. Schultze; mucous cells, Leydig), which do not
belong to the superficial cells but the layer of cells immediately
beneath the horny layer (Schultze, Pfitzner). The cells are rounded
or flask-shaped, and closely resemble the epithelial goblet-cells or
chalice-cells. According to Rudneff they open on the free surface by
stomata; this is, however, denied by Schultze and others.
[Illustration: Fig. 228.
Vertical section through the epidermis from the head of _Rana
esculenta_; after F. E. Schultze. Magnified 600 times.
_A_ Horny layer.
_B_ Middle layer.
_C_ Deep layer.
_D_ Isolated mucous-cells.
]
[Illustration: Fig. 229.
Vertical section through the skin of the back; after Wiedersheim.
_Co_ Superficial layer of cutis.
_Co^1_ Middle layer of cutis.
_Co^2_ Deep layer of cutis.
_D_ Cutaneous glands.
_D^1_ Ducts of cutaneous glands.
_Ep_ Epidermis.
_H_ Horny layer of epidermis.
_P_ Pigment-cells.
_W_ Papillae.
]
According to Pfitzner these cells secrete a substance, which has an
important function in connection with the process of casting the skin,
which consists in the separation of the upper layer from that below
brings about a complete shedding of the skin (Wiedersheim).
[Illustration: Fig. 230.
Surface view of epidermis of _Rana temporaria_; after Eberth.
_A_ Epidermal cell.
_B_ Stoma-cell.
_C_ Attached cuticle from duct of gland.
_D_ Protoplasm and nucleus of stoma-cell.
]
[Illustration: Fig. 231.
Nerve terminations in the branched pigment-cells of the cutis; after
Ehrmann. Hartnack, Obj. 8, Oc. 3.
I. Pigment ceasing abruptly at junction of nerve.
II. Pigment gradually ceasing along the nerve.
]
[The horny layer is, for the most part, very thin, as it consists of
one or two layers of flattened cells only (Schultze), but in some
situations, as on the back and especially on the under surface of the
toes, it is much thickened, and is then rough.
Deeply pigmented, branched cells, capable of contractile movements, are
also found, somewhat sparsely distributed, in the epidermis (Leydig, H.
Müller, Schultze).]
*b.* The *cutis* (Fig. 229, _Co_, _Co^1_, _Co^2_). The epidermis is
generally attached to the corium by means of a continuous layer of
branched cells, which is deeply stained when the animals are fed with
madder (Katschenko). Many of these cells are pigmented. This layer is
seldom flat, but is raised into papillae and folds, which are repeated
by the superimposed epidermis. In addition to this layer the corium
has, except in the webs and supplemental toes, three distinct layers of
connective-tissue, together with much unstriped muscle-fibre (Eberth).
The *superficial layer* (Fig. 229 _Co_) is a loosely-meshed, much
pigmented, vascular layer; it forms a loose support for the numerous
glands, and is traversed by numerous nerves.
The *middle layer* (_Co^1_) forms the groundwork of the cutis; it is
much firmer and more compact than the superficial layer; in section
it appears as a broad band, bounded superficially by a sharp line.
It is chiefly composed of closely packed connective-tissue fibres,
which have, for the main part, a horizontal or wavy course; at certain
points, however, vertical fibres are seen passing towards the surface
and hiding the sharp contour (between _Co_ and _Co^1_). At such points
the two sets of fibres form a sort of basket-work arrangement.
The *deepest layer* (Fig. 229 _Co^2_) is composed of very delicate,
white and yellow elastic fibres, and vessels and nerves; it attaches
the whole integument to the underlying organs. By the looseness of its
structure it forms an important lymph-space.
[The *muscle-fibre* of the cutis is very unevenly distributed. It is
found somewhat freely in the region of the back, the dorsal surface
of the head, and the neck; less freely on the dorsal surfaces of the
extremities, very sparsely on the abdomen, breast, and ventral surfaces
of the extremities; in the feet it appears to be absent (Eberth).
The *pigment* of the skin. As already mentioned pigmented cells occur
in the epidermis, but by far the greater quantity of the cutaneous
pigment is found in the cutis, and more especially in the loose layer
of branched cells, which form the boundary between epidermis and true
cutis. These cells, known as chromatophore-cells, have been carefully
investigated by Harless. They play an extremely important part in
bringing about the well-known changes in the colouring of the skin.
Bimmerman has proved that these cells are influenced by stimulation of
the nerves supplying the region in which they occur. Ehrmann has been
able to trace a direct connection between the nerve-fibres and the
pigment-cells (Fig. 231 I, II). In such cases the pigment was sometimes
sharply marked off, at others was gradually lost along the nerve.]
*c.* The *papillae* and *ridges* of the skin are classified by Leydig
as follows: (1) Small ridges, (2) larger ridges, (3) papillae with
touch-bodies, (4) papillae without touch-bodies, (5) papillae with the
ducts of glands, (6) capillaries in the form of papillae, (7) small
elevations, including the lateral organs.
[All these structures are derived from the cutis, the epithelium above
simply repeating the elevation, and being itself either not increased
in thickness or only to a slight degree. (For further description of
some of these papillae, see Organs of Tactile Sensation, p. 380.)
Peculiar wart-like papillae are found on the articulations of the first
phalanges with the metatarsus, on the same articulations and on the
articulations between the first and second phalanges of the third and
fifth toes, and on the articulations between the first, second, and
third phalanges of the fourth toe. These papillae are connected with
the long flexor tendons by fine tendinous bands (Klein).
*Temporary papillae* are developed in the females of _Rana
temporaria_ during the breeding season; according to O. Huber (_l.
c._), who has described them, their distribution and structure is as
follows:--
During the breeding season the skin of the sides of the trunk of
female specimens of _Rana temporaria_ is rough; the small elevations,
to which this roughness is due, are each about the size of the head
of a pin, and have the appearance of small, rounded, white or pale
rose-coloured, warty growths. The papillae reach their greatest
development and are most numerous on the sides of the trunk (Fig. 232
I); they may be traced on to the tympanic membrane, where they are
arranged in a ring, and in some cases under the eyes to the sides of
the snout.
Anteriorly the papillae are absent from the median portion of the
dorsal surface of the trunk, but posteriorly, behind a line joining
the anterior extremities of the two iliac bones, the papillae cover
the whole surface.
The papillae are large and very numerous around the arms, and pass
by gradual, transitionary stages into the ordinary papillae of the
region. Further they may be traced backwards along the anterior or
extensor surface of the thigh, the upper surface of the leg, foot,
and fifth toe. Occasionally a few papillae were found on flexor
surfaces of the third and fourth metatarsus, and very rarely were any
discovered on the fore-limbs. They occur in no other batrachian of
Germany (Huber).
[Illustration: Fig. 232.
I. Diagram to show the distribution of the temporary papillae in
_Rana temporaria_; after Huber.
II, III. Vertical sections through a temporary papilla; after Huber.
]
Each papilla (Fig. 232 II and III) consists of a hemispherical
elevation of the cutis, and is covered either by a layer of normal
epidermis or in some very rare cases the epidermis was slightly
thickened. The papilla consists of a firm connective-tissue stroma,
and is from 0.2–0.4 mm. in height, 0.4–0.5 mm. broad (after hardening
in alcohol). Such glands as exist in the papilla usually open
laterally (Fig. 232 III), and the blood-vessels are numerous.
The epidermis covering the papilla is well supplied with
nerve-fibres, derived from large ganglion-cells in the papilla
(Fig. 232 III). The branched pigment-cells, so numerous in the
rest of the skin, are absent from the upper parts of the papillae
(Fig. 232 II); the pigment-cells may cease either abruptly or
gradually.
The rose-colour of the papillae is due to the presence of a golden
red pigment. After the breeding season the papillae are gradually
lost, and the pigment-cells wander into the unoccupied parts.
Huber compares the arrangement of the nerves to that of the
‘touch-spots’ of Merkel.
d. The *glands* of the skin are of two kinds, serous and mucous; they
have been fully described by Engelmann and Openchowski.
(1) The *serous glands* (Körnerdrüsen, Engelmann) are large, averaging
in diameter 0.2–0.4 mm., but others vary in size from 0.13–0.8 mm.
in diameter. They are found chiefly on the dorsum of the trunk and
arranged in groups, which vary in dimensions from 3–20 mm. in length
and 2–4 mm. in breadth. They may be found in any part between the
ear-region and the anus, on the dorsal surface of the thigh, especially
towards the outer and inner borders. They are found very sparsely
distributed on the ventral surface of the trunk and on other parts of
the extremities, though as a rule a few are found on the dorsal surface
of the feet and near the phalanges. They are wanting in the nictitating
membrane.
The glands have a rounded form, with the ducts placed at their
superficial poles. The glands possess three coats; externally is a
coat of connective-tissue, the fibres of which are arranged closely
together, and cross one another at acute angles: these fibres are
continuous with the connective-tissue of the surrounding cutis. Inside
this coat is a thick layer of muscle-fibre (0.005–0.015 mm.); the
muscle-cells are arranged meridionally, with the one pole towards the
superficial surface of the gland, the other pole at the deep surface
(Hensche). Many of the fibres, however, extend only one-fourth or
one-third of this distance.
The innermost coat is a simple layer of epithelial cells, which rests
directly on the muscle-layer; no basement-membrane or space of any
kind having been discovered between the two layers. When examined in
the recent state these cells are conical or cylindrical in shape,
and have the appearance of goblet-cells: they possess a delicate but
distinct cell-wall, but are open towards the lumen of the gland. One or
sometimes two rounded nuclei are found towards the base of the gland,
and surrounded by a small amount of granular protoplasm; the rest of
the cell is occupied by a mass of small, rounded, highly refractive
granules of about 0.002–0.01 mm. in diameter. These granules are also
to be found in the protoplasm of the cell; they contain a substance
capable of swelling freely, and present many points of resemblance to
the granules found in the cells of the oviducts.
These glands represent the poison-glands or lateral glands of toads,
and the ear-glands or parotids of Caecilia (Leydig). According to
Leroux[81], Gratiolet and Cloëz[82], the reaction of these glands in
toads varies; according to du Bois-Reymond[83] the reaction is, in
frogs, acid; Engelmann[84] found the reaction of these glands, in
frogs, to be neutral, frequently, however, with a tendency to either
acidity or alkalinity.
These glands correspond with the ‘large glands’ of Hensche, the
‘contractile glands’ of Stieda, the ‘large, dark glands’ of Eberth,
and the ‘large contractile glands’ of Leydig’s earlier publications.
[Footnote 81: Journal de Médicine, Vol. XI, p. 75.]
[Footnote 82: Comptes rendus, 1851, Vol. XXXII, p. 582.]
[Footnote 83: Untersuchungen über thier. Electricität, 1830, Vol. I,
p. 17.]
[Footnote 84: Arch. f. d. ges. Physiol., 1852, Vol. V, p. 505.]
(2) The *mucous glands* are, as a rule, smaller, more numerous, and
more evenly distributed than the serous glands. Usually they lie in
such close proximity as almost to touch one another; according to
Engelmann an average number of sixty is found in one square millimeter;
on the abdomen 62–68; flexor surfaces of the posterior extremities in
some places 76, but in others only 30–40; on the webs only 2–6 to the
square millimeter. On the nictitating membrane they are arranged in two
or three parallel rows, are large, and closely applied to each other.
They are wanting on the deeper surface of the nictitating membrane.
The glands are generally rounded in form and have a short neck
(Fig. 229). Glands in the recent condition, and not fully contracted,
have a diameter of 0.06–0.21 mm., the majority measure 0.12–0.16 mm.
in diameter. The upper pole of the gland is usually placed directly
under the boundary between epidermis and cutis, and, as a rule, about
0.06–0.1 mm. beneath the superficial surface of the skin. The duct
usually passes directly to the surface, in a straight line.
The structure of the glands corresponds, in general, with that of the
serous glands. Externally is a coat of connective-tissue, which is,
however, more firmly attached to the muscular coat than in the case
of the serous glands, and may easily be mistaken for a structureless
basement-membrane[85] (Eberth). The muscular coat is very thin, and
consists of 16–20 flat, spindle-shaped fibres; extending from the
upper to the lower pole as in the serous glands. At a short distance
(0.015–0.022 mm.) from the upper or superficial pole each fibre has an
ellipsoid thickening, composed of granular protoplasm, and containing a
clear oval nucleus.
[Footnote 85: Openchowski (_l. c._) describes a basement-membrane.]
The inner coat is composed of a single layer of epithelium, placed
directly on the muscular coat. The epithelium has a thickness of about
0.01–0.02 mm.; it is, however, usually thicker towards the base than
towards the duct. The boundaries between the cells are very indistinct,
and so give the epithelium the appearance of a continuous layer of
protoplasm. The glandular epithelium is not directly continuous with
the epithelium of the duct.
The shape of the cells depends upon their physiological condition.
In a state of semi-contraction of the gland, the cells are cubical,
and their free borders pushed into the lumen in a slight convexity;
when the gland is fully distended the cells are two or three times as
broad as deep; in a fully contracted gland the cells are conical or
cylindrical. In the first-mentioned condition their diameter varies
from 0.006–0.018 mm. The larger glands are estimated to possess about
150 cells, the smaller only thirty to forty (Engelmann).
The cells appear to have no cell-wall, certainly they possess none
on their free surfaces. Their protoplasm distends very freely on the
addition of water. As a rule some cells are much more granular than
others of the same gland; each cell has a rounded oval, clear nucleus,
and vacuoles are frequently met with.
The lumen of the gland contains a colourless, transparent, watery
fluid, which contains mucous material.
The ducts of the glands are thick-walled cylinders, circular in
section. Their diameter varies from 0.02–0.05 mm., their length is
equal to the depth of the superimposed epidermis. The wall of the duct
is lined with two or three concentric layers of highly refractive,
nucleated cells. The cells are arranged parallel to the length of the
duct: in transverse section they are sickle-shaped or semilunar. In
the upper third or fourth of the duct the inner wall is lined with
a cuticle, very resistant to the action of acids or alkalies: it is
thrown off with the skin in the process of casting. In the horny layer
of the epidermis this cuticle is partially closed by a stoma-cell
(Eberth), (Fig. 230), which possesses a tri-radiate opening.
The glands correspond with the ‘non-contractile glands’ of Hensche,
the ‘dark and bright glands’ of Stieda, the ‘glandule mezzane,
piccole e piccolissime (ad epitelio pavimentoso)’ of Ciaccio, and the
‘small, dark, and medium-sized, bright glands’ of Eberth.
*e.* The *peculiarities* of the *skin* of the *supplemental toe* and
*webs*. The supplemental toe presents, in the male, a rounded, oval
swelling (Fig. 234 I, _D_), with the end of the digit projecting from
the distal end.
Normally this swelling is usually of a grey colour; during the breeding
season, however, the swelling enlarges, and becomes of a black or deep
brown colour (Leydig).
[Illustration: Fig. 233.
Vertical section through the epidermis of the supplemental toe of _Rana
esculenta_; after E. Schultze. Magnified 400 times.
]
The epidermis covering this swelling (Fig. 233) is much thickened
and produced into projecting papillae. The cutis also shows a marked
thickening and an increase in vascularity; the glands are of the kind
described as mucous glands (Engelmann). They present, however, an
immense increase in size, and at the same time are so closely applied
to one another as to form the main part of the enlargement (see also
Organs of Tactile Sensation).
[Illustration: Fig. 234.
I.
Fore-foot of a male frog.
_D_ Glandular swelling on the supplemental toe.
_W_ Warty papillae on the inter-phalangeal joints.
II.
Vertical section through the swelling on the supplemental toe of a male
frog.
_B_ Blood-vessels.
_Co_ Cutis.
_D_ Glands.
_Ep_ Epidermis.
_M_ Muscles.
_P_ Pigment-cells.
]
In the *webs* the various layers of the corium are more or less fused
together, and cannot be distinguished from each other; the glands are,
as a rule, smaller than on the general surface of the skin.
*f.* The *blood-vessels* and *lymphatics* of the skin.
(1) The *blood-vessels* of the skin are arranged in two networks; the
one is situated immediately beneath the epidermis, and consists of fine
capillaries, forming a fine-meshed network, which enclose the ducts of
the glands. The meshes vary considerably in size and shape (Hyrtl and
Langer). The second network of vessels is situated under the cutis; the
vessels forming it are small, and the meshes formed are large. From
this network vertical branches pass to form the superficial network;
these branches usually course along the strands of vertical fibres
already described, and break up to form capillaries only when they
reach the deeper surface of the epidermis (Langer), (Fig. 235 I).
[Illustration: Fig. 235.
The blood-vessels and lymphatics of the skin; after Langer. Arteries
striped, lymphatics shaded.
I. Vertical section through skin of thigh.
II. Horizontal view of arteries and lymphatics of the skin.
III. Peculiar tortuous arteries of the lamina inguinalis.
IV. Arteries and lymphatics of the web.
]
In _some situations_, however, the vertical branches divide earlier;
this is especially the case in parts where the glands are very closely
packed, as in the nictitating membrane, the upper lip, and the
swelling of the supplemental toe (in the male). In these parts the
division takes place at the deeper surface of the glands (Langer).
A _peculiar arrangement_ of the cutaneous blood-vessels is found in
the lamina inguinalis, where a chain of reddish points can be made out
with the naked eye; examined more carefully (Fig. 235 III) each reddish
point is found to be a complex loop of capillaries. They extend from
the skin to the hinder lymph-hearts, where they form a close network
which partially invests the lymph-hearts (Langer).
(2) The *lymphatics of the skin* (see also p. 252), like the arteries,
reach the deeper surface of the epidermis by coursing along the bands
of vertical fibres; they then form a network of capillaries with
rounded meshes (Fig. 235 I, II, IV), and lying immediately under the
epidermis, and a network which surrounds the various glands. The
lymphatic-capillaries are as a rule larger than the blood-capillaries.
The lymphatic network lies beneath the blood network, and the vessels
of the two systems branch independently (Langer).
In those parts where the glands are placed closely together this
arrangement is modified, and corresponds with the modifications found
in the blood-capillaries in these regions. The vertical branches divide
so as to form a network below the glands, and from this vertical
branches pass in the same direction between the glands to form a
secondary network on the superficial surface of the glands (Langer).
g. The *nerves* of the skin. The cutis is very richly supplied with
both medullated and non-medullated nerves. In the subcutaneous tissue
the nerves destined for the skin branch freely to form numerous fine
twigs, which, without actually inosculating, form a fine meshwork.
From the larger trunks numerous vertical branches pass vertically to
form a secondary network, from which both coarse and very fine twigs
pass to encircle the glands. The fine fibres which supply the glands
are non-medullated and possess oval nuclei; their diameter varies from
0·001–0·002 mm. The number of nerve-fibres is not much smaller than
that of the muscle-fibres (Engelmann). (See also Organs of Tactile
Sensation.)]
II. THE ORGANS OF TACTILE SENSATION.
The organs of tactile sensation are the nerve-plexus of the epidermis,
the touch-spots of Merkel, the lateral sense-organs, and the
touch-corpuscles of the tendons.
*a.* [The *nerve-plexus* of the epithelium is very incomplete as
compared with the plexus found in the epithelium of the cornea; here
and there a few non-medullated fibres have been traced a short distance
between the epithelial cells. No such mode of nerve-terminations in the
epithelium as is described by Eberth and Macullum in the tadpole can be
found in the adult.]
*b.* The *touch-spots*. What knowledge we possess of these organs is
due to the labours of Leydig, Ciaccio, Eberth, and more particularly
those of Merkel.
At the base of each papilla, which contains such a touch-organ, is
found a number of colourless, flattened cells, arranged either in a
single layer, or heaped up and connected with very fine nerve-fibres.
The flat surface of the cells is parallel to the surface of the body,
and they do not form a separate or circumscribed body, they are
therefore better spoken of as touch-spots than as true touch-bodies
(Merkel). The larger organs are found in the papillae, but smaller ones
may be found on any part of the skin.
[Illustration: Fig. 236.
I. Lateral sense-organ of tadpole of frog. Half-schematic.
_c_ Central zone (nerve-epithelium).
_Ml_ Membrana limitans.
_p_ Peripheral zone (sustentacular cells).
_R_ Hyaline tube.
II. Touch-corpuscle from the sheath of a digital tendon of the frog;
after Löwe. Schieck, Oc. O., Obj. 9. ]
These organs are best seen in the prominence or swelling upon the
supplemental toe during the breeding season; they are numerous on the
dorsal surface of the trunk, but occur most frequently on the under
surface of the hinder feet (_Planta pedis_). After the breeding
season the organs of the thumb undergo a retrogressive change, which
results in a network of spindle-shaped and branched cells with fibres
(Wiedersheim).
*c.* The *lateral sense-organs* attain their highest development in the
tadpole; in the adult the organs have undergone a retrogressive change,
the result of which is that the organs are diminished in size; the
whole organ sinks by the formation of a tube, which is then closed by
a mucous secretion, consequently the organ is functionless: with this
loss in function and change in position of the organs a corresponding
diminution of the ramus lateralis nervi vagi occurs (Merkel).
Each lateral organ (Fig. 236 I) consists of a slightly elevated
papilla, with the centre depressed, and in each may be distinguished a
central and a peripheral zone (_c_, _p_). The central zone consists of
a group of pear-shaped cells, with the narrower ends directed towards
the free surface; each is connected below with a nerve-fibril, while
above it bears a short, stiff cilium. The cilia are enclosed in a
delicate hyaline tube (_R_), which is open superficially, closed by the
papilla below; consequently the cilia are in direct contact with the
surrounding medium. The peripheral zone is a layer of pale, flattened,
cylindrical cells (_p_); they possess, at their upper borders, a
perforated _membrana limitans_ (_Ml_), through which the cilia pass.
These cells serve as a support to the central cells, and are themselves
surrounded by ordinary epithelial cells.
The distribution and arrangement of the lateral organs is alike in
fish and larval amphibia. They are most numerous on the head, where
they surround the eye, and are continued forwards to the snout and
on to the lower jaw. All these organs on the head are in connection
with the trigeminal nerve. A line of these organs passes from the
hinder part of the circumference of the eye along the gill-cover to
the neck, where the lines of opposite sides are usually, though not
constantly, joined by a transverse line of the same organs; thence
the lines are continued along the trunk to the tip of the tail. In
fish there exists, as a rule, only one pair of such longitudinal
lines (_Linea lateralis_); in anura-larvae, proteus, and in all
salamanders there are three pairs: of these one pair lies near the
vertebral column, the second at the junction of the flank with the
abdomen, and the third corresponds to the lateral line in fish
(Malbranc).
At an early developmental period each metamere possesses one pair of
such lateral organs; in later life, however, this simple arrangement
is lost, and each segment usually possesses a group of organs.
The organs of the trunk and tail are supplied by branches of the
Ramus lateralis nervi vagi. Both in fish and amphibia a reparative
process, by means of fission, occurs; as a consequence the organs are
found in various stages of development.
*d.* [The *touch-corpuscles* of Golgi and Löwe (Fig. 236 II) are found
on the joints of the digits. They are surrounded by connective-tissue
and are spindle-shaped; the two ends of the spindle are glassy in
appearance, the middle portion is fibrous. In the thickest part of the
corpuscle is a zone of nuclear bodies.
They are regarded as touch-bodies (Golgi and Löwe). Golgi (_l. c._)
describes two kinds of touch-organs in connection with muscle and
tendon; one class correspond with those just described, he names them
‘nervous muscle-tendon organs.’ The second class he compares with the
touch-bodies found in the conjunctiva.]
III. THE ORGANS OF TASTE.
The organs of taste are not confined to the surface of the tongue,
but are also found on the mucous membrane of the roof of the mouth,
especially in the neighbourhood of the vomerine teeth; on both roof and
floor of the mouth these organs can be traced to the commencement of
the oesophagus.
[The mucous membrane of the tongue possesses two kinds of papillae and
numerous glands.
*a.* The *filiform papillae* are the more numerous; they are conical
or thread-like in form, and consist of connective-tissue, with a few
striated muscle-fibres; they include blood-capillaries, but no nerves
have been traced into them (Leydig). They are covered with ciliated
epithelium and goblet-cells.
*b.* The *fungiform papillae* are much larger, and are paler than the
filiform papillae. The free ends of these papillae are broader than
the bases, and when the epithelium is removed the free ends present a
concavity. From the base to the border of the free surface each papilla
is covered with the usual oval, ciliated epithelium, but at this border
an abrupt transition takes place. The epithelium covering the end of
the papillae is of three kinds: goblet-cells, cylindrical cells, and
forked cells; the three forms of cells are quite characteristic, and no
intermediate transitionary forms are met with.
(1) The *goblet-cells* (Fig. 237 I) are arranged vertically to the free
surface of the papilla; they are from 0.020–0.024 mm. in length, and
from 0.01–0.02 mm. in diameter. In the lower third of the cell is found
a nucleus of about 0.008 mm. in diameter, and this encloses a nucleolus
of about 0.001 mm. in diameter. Close underneath the nucleus the cell
is contracted to form an irregular process or foot. The contents of
the cells are a very finely granular, transparent protoplasm. These
cells form the outermost part of the epithelium, covering the end of
the papilla; all the cells belonging to the same papilla are of the
same size. In consequence of the mutual pressure which they exert on
each other they present, in transverse section, six-sided outlines
(Fig. 237, 2). The nuclei of adjacent cells are placed at almost
exactly the same level. By the action of reagents on these cells
very remarkable forms are obtained, in consequence of the protoplasm
swelling and being forced upwards (Fig. 237, 5).
[Illustration: Fig. 237.
I. Various parts from the fungiform papillae; after Engelmann.
1. Nerves of a fungiform papilla. Magnified 450 times.
2. Surface view of the epithelium, after five minute’s action of
iodized serum. Magnified 600 times.
3. Goblet-cell with adjacent forked cell. Magnified 450 times.
4. Portion of the papilla, after removal of the goblet- and forked
cells; only cylindrical cells remaining. Magnified 400 times.
5. Goblet-cell, with swollen contents. Magnified 450 times.
6–13. Various forms of isolated forked cells. Magnified 450 times.
II. Sections of two glands of the tongue; after Biederman.
1. Resting-gland.
2. Gland after stimulation.
]
(2) The *cylindrical cells* (Fig. 237 I) have rounded free extremities,
which reach to the general surface of the epithelium; the remainder
of the cell is cylindrical except towards its lower extremity, where
it is slightly dilated and encloses an oval nucleus: the protoplasm
of the cells is very finely granular. These cells rest on the
connective-tissue of the papilla, and are in close juxtaposition, so
that several hundreds of them are found on one papilla.
The spaces between the upper parts of the cylindrical cells are
occupied by the goblet-cells and by the forked cells.
(3) The *third form* of cell found on the fungiform papillae are named
*forked cells* (Fig. 237 I) by Engelmann. The body of the cells has
an ellipsoidal form, is from 0.006–0.008 mm. in its longer diameter,
and 0.003–0.004 mm. in the shorter diameter. Processes arise from both
poles. The peripheral processes arise by a short common stem, which
then divides into two or rarely three branches; the whole process is
from 0.004–0.008 mm. in length, and is always just sufficiently long
to reach the general surface of the epithelium: when the peripheral
process is long the central is usually shorter, and _vice versâ_.
The central processes arise by a stem from 0.001–0.002 mm. in
thickness, which usually divides dichotomously. The length of this
process may be as much as 0.025 mm. or almost nil.
The forked cells are about twice as numerous as the goblet-cells, they
occupy the spaces between the cylindrical cells and the goblet-cells.
The branched central processes form a network on the connective-tissue
of the papilla, which is, at this point, perforated by a rich plexus
of fine non-medullated nerve-fibrils. It is not decided whether the
processes of various cells inosculate. Engelmann regards these forked
cells as the taste-cells and as the sensory nerve-endings.
The *bodies* of the fungiform papillae contain blood-vessels and
nerves, the latter enter as medullated nerves, but lose the medulla,
somewhat abruptly towards the upper end of the papilla, where they form
a sort of nerve-cushion (Nervenkissen, Engelmann).]
Taste-organs are also found on the roof and other parts of the mouth,
and present many points of resemblance to the lateral sense-organs.
Like them they present a peripheral zone and a central zone; the cells
of the latter, however, are not longer than the peripheral cells, and
possess no cilia (J. van der Hoeven, Merkel).
IV. THE NOSE.
The *anterior nares* or nostrils are two small openings, placed
directly in front of the anterior angle of the eye; the distance
between the eye and the corresponding nostril being, in the case of
adult animals, about five or six millimeters. The anterior nares are
surrounded by rims, which are contracted below, and so form very short,
tentacular-like prominences.
The *posterior nares* (choanae) are about four millimeters from
the anterior nares, consequently the long axis of the nose is,
approximately, of this length. (For the external muscles of the nose,
see p. 59.)
The *boundaries* of the *nasal cavities* are as follows:--the roof of
each cavity is formed by the dorsal plate of the sphenethmoid, the
nasal bone, and the premaxillary bone; the floor is formed by the vomer
and the palatine bone, the inner wall by the vertical septum of the
sphenethmoid, the outer wall by the premaxillary and maxillary bones,
the anterior wall by the premaxillary bone, and the posterior wall by
the sphenethmoid. The cartilaginous portion of the nasal skeleton (see
also p. 27) projects into the general cavity and subdivides it into
various sinuses, which have been the subject of careful investigation
by Born and Wiedersheim.
*a.* The *nasal cavities* are best examined by means of serial
transverse sections; in a section through the anterior nares
(Figs. 238, 239) there will be seen three sinuses on either side:--
(1) The *superior sinus* (_on_) is large, rounded, and placed against
the cartilaginous septum; the sinus is lined with olfactory epithelium.
Posteriorly the sinus extends beyond the posterior nares, and is
bounded by the anterior surface of the sphenethmoid. In front it ends
in a rounded concavity, likewise lined with olfactory mucous membrane.
Opposite the anterior nares the cavity is partially subdivided, by a
longitudinal process on the floor, into two cavities; the anterior
nares open into the external chambers, and therefore not directly into
the main cavity of the superior sinus. The superior sinus communicates
with the inferior sinus by a narrow, almost vertical, slit.
(2) The *inferior sinus* (_un_) is narrow and flattened from above
downwards; it is placed against the maxillary bone, and represents the
maxillary sinus of the frog. Externally it possesses a descending arm.
The inferior sinus is continued backwards and opens on its inner side
into the posterior nares, or rather into the mouth. The descending arm
extends only a short distance forwards, to terminate near the point
where the superior and inferior sinuses communicate by only a narrow
opening. This sinus is on the whole broader in front than behind, and
has a general direction from without, inwards and forwards underneath
the superior sinus.
[Illustration: Fig. 238.]
[Illustration: Fig. 239.
Frontal sections through the nose of two tadpoles; after G. Born.
_A_ Anterior naris.
_b_ Cartilaginous floor.
_C_ Cutis.
_d_ Cartilaginous roof.
_Ep_ Epidermis.
_gi_ Intermaxillary gland.
_gni_ Lower nasal gland.
_gns_ Upper nasal gland.
_K_ Maxillary sinus.
_l_ Concha narium (os lachrymale).
_m_ Maxillary bone.
_MS_ Oral mucous membrane.
_ok_ Cartilage.
_on_ Upper blind sac.
_R_ Pharyngeal gland.
_S_ Nasal septum.
_se_ Septum between the upper and lower
nasal cavity.
_sn_ External blind sac.
_T_ Trabeculae.
_Th_ Lachrymal duct.
_un_ Lower blind sac.
]
(3) The *lateral sinus* (_sn_) is situated in the partition between
the superior and inferior sinuses, or rather between the external
chamber of the upper sinus and the inferior sinus. The lateral sinus
is triangular in form, being narrow in front and broad behind; at its
inner posterior angle it opens on the free border of the horizontal
partition between the superior and inferior sinuses; this opening is,
however, continued along its roof, so that the lateral sinus opens also
into the superior sinus.
*b.* The *nasal cartilages*. The two nasal cavities are completely
separated by the cartilaginous septum (Figs. 238 and 239 _S_), and
are for the most part lined by cartilage. The anterior end is formed
of concave cartilages, while the posterior, being situated in the
sphenethmoid, is usually more or less ossified in the adult. The
posterior wall has two openings: a larger near the septum for the
olfactory nerve, and a smaller, more externally, for the nasal branch
of the trigeminal nerve.
The anterior wall is more complex than the posterior, being thicker in
the middle than at the sides, and possessing three blind sacs for the
three sinuses. From the anterior wall two processes project backwards
between the sacs and enclose them more or less completely.
Of the three sacs or cavities only the lower is completely surrounded
by cartilage. The upper sac is in part bounded by a shell-shaped,
concave cartilage (_Concha narium_, of various authors; _Os
lachrymale_, Born), which covers it anteriorly and externally, and is
attached by a small base to the roof of the lower sac (Fig. 239, _l_),
its upper border bounding the anterior naris (Fig. 15). The upper and
external parts of the upper sac have no cartilage. The cartilages of
the outer wall of the lower, blind sac extend backwards to the point
where the maxillary sinus commences to descend: the roof, however,
is prolonged further backwards by two small cartilaginous processes;
the inner is short, the outer forms the roof of the descending arm of
the sinus, and joins a cartilaginous process, which commences at the
anterior portion of the roof of the nasal cavity, passes backwards and
downwards to a broad plate, and forms an incomplete outer wall to the
nasal cavity. There are also three cartilaginous processes--α. One
arising from the outer side of the floor at the level of its junction
with the anterior wall; it is a flattened process and passes outwards,
and bifurcates at its end to meet the premaxillary and maxillary bones
(this is the Oberkieferfortsatz of Ecker), (p. 28, Fig. 14 _n″_). β. A
delicate process on either side, described by Wiedersheim (see p. 280).
γ. The third pair of processes are fully described for the first time
by Born; each arises at the lower border of the corresponding _Concha
narium_, passes forwards and downwards under the ascending process of
the premaxillary to the point where process β is attached to this
bone. The processes are flat and approach one another obliquely; in
older animals they are united at the point of contact.
At the junction of the anterior wall, the septum, and the floor is a
large aperture in the cartilaginous capsule, through which passes the
chief nasal branch of the trigeminus, which supplies numerous branches
to the intermaxillary gland.
*c.* The *glands* of the *nasal region* are Bowman’s glands, the
intermaxillary glands (Wiedersheim), the lower nasal gland (Born),
the upper nasal gland (Born), the pharyngeal gland (Born), and the
lachrymal duct.
[Illustration: Fig. 240.
I. A. Bowman’s glands in situ from _Rana temporaria_; after C. K.
Hoffmann. Magnified 150 times.
B. Section of Bowman’s gland; after C. K. Hoffmann. Magnified 300
times.
II. Vessels of nasal mucous membrane of _Rana esculenta_; after Langer.
Blood-vessels striped, lymphatics shaded.
]
(1) The *intermaxillary gland* (see p. 280).
(2) The *lower nasal gland* is placed along the septum and extends
as far back as the posterior nares, opening by its ducts (Figs. 238
and 239 _gni_) into the inferior blind sac of the nasal cavity. In
histological structure it corresponds with the intermaxillary glands,
except that the glandular tubes are somewhat smaller, and the glandular
epithelium stains somewhat less easily with carmine (Born).
(3) The *upper nasal gland* occupies the space between the _Concha
narium_ (_Os lachrymale_, Born) and the neighbouring cartilages, it
also surrounds the anterior naris and the opening of the lachrymal
canal (Figs. 238 and 239 _gns_). The numerous ducts open on the mucous
membrane covering the oblique cartilage (above described, as passing
from in front, downwards and backwards, and forming an incomplete outer
wall of the cavity), and its hinder prolongation.
(4) The *pharyngeal gland* is placed transversely behind the posterior
nares, and surrounds the vomerine teeth. A portion of the ducts open
into the posterior nares, the remainder on the mucous membrane of the
pharyngo-oral cavity at two symmetrically placed points (Fig. 238 _R_).
(5) The *lachrymal duct* (see p. 428) opens into the nasal cavity at
the point where the lateral sinus opens into the external chamber of
the superior nasal sinus.
(6) [The *glands* of *Bowman* (Fig. 240 I) are freely distributed
in the nasal mucous membrane. Each gland is usually rounded or
flask-shaped, and consists of a single layer of large epithelial cells
possessing distinct nuclei and nucleoli. The epithelium of the glands
possesses no basement membrane (M. Schultze and Hoffmann[86]), but is
bounded externally by a layer of nerve-fibres and connective-tissue.
[Footnote 86: Paschutin describes a basement membrane.]
The glands situated more superficially have straight ducts opening
on the surface; those placed more deeply have usually curved ducts
(Paschutin).]
*d.* The *mucous membrane* of the nasal cavities. That part of
the superior sinus immediately around the anterior naris is lined
with stratified epithelium; the rest of the nasal cavity is lined
with columnar ciliated epithelium The epithelial layer rests on a
subepithelial network, and this again on a submucous connective-tissue
layer possessing numerous vessels and nerves.
(1) The *epithelial layer* (Fig. 241) consists of columnar cells,
which in the olfactory region are of two kinds; in other regions
the epithelium consists of closely-applied ciliated, thick cells,
possessing large oval nuclei, and having irregular, branched bases or
‘feet.’ The cells are 0.032–0.048 mm. long; the nuclei 0.016–0.018 mm
long, and 0.006–0.008 mm. broad. The free borders of the cells bear a
number of fine cilia.
In the olfactory region a second set of cells, olfactory cells, are met
with; these cells (Fig. 241 II) possess each an oval body, enclosing
a large nucleus, and a peripheral and central process. The peripheral
processes reach to the general surface of the surrounding epithelium
and there terminate in a number (5–8) of stiff cilia; these cilia are
sometimes 0.09 mm. long, they are thicker at their bases than the
ordinary cilia (Schultze); according to Hoffmann, a second system of
stiffer and longer cilia is met with; of which each olfactory cell
possesses as a rule only one. The bodies of the olfactory cells are
0.009–0.010 mm. long, and 0.007–0.008 mm. broad; the peripheral
processes vary considerably in length, according to the position of the
body (0.03–0.05 mm.). The central processes vary from 0.02–0.03 mm.
in length. The peripheral processes are considerably thicker than
the central processes. No _membrana limitans olfactoria_ has been
discovered in the frog.
(2) The *second layer* consists of a network of processes belonging
to the bases of the central processes of the superimposed epithelial
and olfactory cells. Numerous highly refractive nuclei possessing
nucleoli are situated in the meshes of this network; they correspond in
all particulars with the nuclei of the olfactory cells (M. Schultze).
According to Exner the branched processes of the epithelial cells and
of the olfactory cells unite to form a complex plexus. Other observers,
Paschutin, Cissoff, v. Brunn, Schultze, and Hoffmann oppose this view,
and hold that the central processes of the olfactory cells do not
unite with the processes of the epithelial cells, but that they are in
direct continuity with the fibrils of the olfactory nerve. This view is
probably correct.
[Illustration: Fig. 241.
Separations from the olfactory mucous membrane of _Rana temporaria_;
after C. K. Hoffmann.
I. Surface view of the olfactory mucous membrane. Magnified 350 times.
II. Epithelial and olfactory cells. Magnified 600 times.
III. Epithelial cells. Magnified 600 times.
]
The submucous layer has a loose connective-tissue matrix, which
encloses the glands, nerves, and vessels; according to Paschutin
two pigment-layers may be distinguished: the one, immediately under
the middle layer of this mucous membrane, is continuous and deeply
pigmented; the second is deeper and does not form a continuous layer.
This layer is extremely rich in blood-vessels and lymphatics (Langer
and Paschutin), (Fig. 240 II). The lymphatics are relatively very large
and are very numerous (Langer).
In this layer the central processes of the olfactory cells form bundles
of fibres, lying parallel with the surface of the mucous membrane
(Paschutin, Cissoff, and Schultze).]
V. THE EAR.
(Re-written by the translator from Das Gehörorgan der Wirbelthiere,
by G. Retzius, 1881.)
The organ of hearing is divisible into two parts, the tympanum or
middle ear, and the labyrinth or internal ear; an external ear is
absent, unless a very slight depression of the tympanic membrane be
regarded as such.
A. The *tympanum* (_Cavum tympani_) is a cavity, bounded externally by
the tympanic membrane and internally by the capsule of the internal
ear; it communicates by means of the Eustachian tube (tuba Eustachii)
with the pharyngo-oral cavity (Fig. 178).
*a.* The *tympanic membrane* lies immediately underneath the skin, but
can be separated from that structure; externally it possesses a very
slight depression, the only trace of an external ear found in the frog.
The tympanic membrane is of a rounded oval form, being a little wider
in the transverse than in the longitudinal diameter; the membrane is
directed outwards.
After detaching the skin (Fig. 242 I) the membrane is seen to be
attached by its circumference to a ring of cartilage (_Annulus
membranae tympani_; see also p. 26) (_at_); the ring is attached
anteriorly and above to the squamosal bone (_sq_), and in the rest
of its circumference to the surrounding soft parts, _i.e._ the _M.
depressor maxillae_ (_dm_), the _M. temporalis_ (_t_), and their
fasciae. In the middle of the membrane is a small, rounded, white
part (_col_) which can be traced backwards and upwards towards the
circumference.
The membrane consists of fibres of connective-tissue which radiate
peripherally from the central point of attachment of the columella;
in the peripheral portion unstriated muscular fibres are also found
(Leydig).
[Illustration: Fig. 242.
I. The tympanic membrane of _Rana esculenta_; natural size. After
Retzius.
II. The tympanum as seen after removing the tympanic membrane; natural
size. After Retzius.
_at_ Annulus membranae tympani.
_col_ Columella.
_ct_ Fissure-like cavity.
_dm_ M. depressor maxillae.
_mt_ Tympanic membrane.
_sq_ Squamosal.
_t_ M. temporalis.
]
Internally the tympanic membrane is covered by columnar epithelium, a
continuation of the mucous membrane lining the tympanic cavity.
*b.* The *tympanic cavity* (Fig. 242 II) is seen, after removing the
tympanic membrane, as a flattened, funnel-shaped cavity. The walls of
the cavity are lined with a pigmented mucous membrane, under which the
cartilaginous ring (_annulus membranae tympani_) extends internally
to form the greater part of the wall of the cavity. The cavity is an
elongated oval slit (Fig. 242 II, _ct_), with its longer axis directed
from above and in front, downwards and backwards, and leading inwards;
it is bounded above, below, and in front by the squamosal, behind by
the soft parts; above in the roof is the cartilaginous part of the
_columella_. The inner or deeper portion of the tympanic cavity is
bounded in front by the squamosal, and by the squamous process of the
prootici (_proc. squamosus prootici_) in front and above; internally
by the cartilage (primordial-cranium) between the prootic and the
exoccipital; behind by the _M. depressor maxillae_. This part of the
cavity is rounded and covered with mucous membrane, and has in its
roof the bony part of the columella, which covers the _foramen ovale_
(_fenestra ovalis_) by its oval, widened end-piece. This deeper portion
of the cavity communicates by a short, wide Eustachian tube with the
pharyngo-oral cavity; the tube is wide and is of a rounded oval form in
section: anteriorly, externally, and internally it is bounded by the
pterygoids, posteriorly by soft parts, in which is embedded the styloid
process.
The tympanic cavity can be examined from without after removing the
tympanic membrane, or from below by means of the Eustachian tube.
*c.* The *columella auris* (Figs. 12, 243) is described by Retzius as
consisting of three portions, of which the middle is bony, the external
and internal cartilaginous. Parker divides it into four parts (see
pp. 25, 26).
The *extrastapedial* (Fig. 243 _a′_, _a″_) is attached to the middle
of the tympanic membrane by the oval surface opposite _a′_, and is
attached by the process _a‴_ to the _annulus tympanicus_; this process
of Retzius is the *suprastapedial* of Parker.
The *mediostapedial* (Parker), or middle bony piece of Retzius
(Figs. 12 _a′_, 243 _b_, _b′_), is narrow externally but widens
internally to articulate with the interstapedial. Just before reaching
the latter it gives off a process (_b′_), to which are attached a few
fibres of striated muscle (_m_).
[Illustration: Fig. 243.
The columella; after Retzius. Magnified eight times.
A. Seen from above.
B. Seen from behind.
_a′_ Extrastapedial (Parker). Outer cartilaginous portion
attached to middle of tympanic membrane (Retzius).
_a″_ Attachment of extrastapedial to mediostapedial.
_a‴_ Suprastapedial (Parker). Portion embedded in mucous
membrane (Retzius).
_b′_ Mediostapedial (Parker). Bony part (Retzius).
_b″_ Process of mediostapedial.
_c_ Interstapedial (Parker). Inner cartilaginous piece
(Retzius).
_m_ Insertion of small muscle.
]
The *interstapedial* (Figs. 12 _a_, 243 _c_) is cartilaginous; it is
thick with a sharply cut-off, slightly concave end, which is placed
against the _fenestra ovalis_. The inner surface of the interstapedial
is, however, distinctly larger than the opening of the _fenestra
ovalis_, consequently it does not fit in accurately, but is attached to
the border of the opening by means of connective-tissue; the margins of
the _fenestra ovalis_ are hollowed (Fig. 245 II) so as to form a _fossa
fenestrae ovalis_, and it is really to the margin of this fossa that
the connective-tissue capsule of the interstapedial is attached.
B. The labyrinth or internal ear is contained in a capsule formed of
bone and cartilage.
*a.* The *capsule of the labyrinth* (Fig. 244) is formed of two bones,
the prootic and the exoccipital (according to Hasse this includes
the opisthotic), which are united by cartilage belonging to the
primordial-cranium. The fronto-parietal, squamosal, and parasphenoid
take only an indirect part in its formation.
On the whole the anterior half of the capsule is formed by the prootic,
and the hinder half by the exoccipital; the cavity has its long axis
directed from within and above, downwards and outward; the cavity is
comparatively large and rounded, and contains the membranous ear.
Four surfaces can be distinguished: a supero-external, an
interno-inferior, an anterior, and a posterior (Retzius).
(1) The *supero-external surface* is divided into two parts by a
strong, transverse, bony ridge, the _processus squamosus prootici_.
The upper half is concave, is directed upwards and outwards, and is
formed by the prootic; it is separated from the anterior surface by
a bony ridge, which marks the position of the anterior semicircular
canal. Posteriorly and medianly it is separated from the posterior
surface by a cartilaginous ridge, running from above, downwards and
outwards, which marks the position of the posterior semicircular canal.
The cartilaginous hinder root of the _processus squamosus prootici_
arises in the cartilaginous ridge just mentioned, and runs outwards and
forwards, covering the external semicircular canal.
[Illustration: Fig. 244.
Antero-posterior section through the capsule of the right labyrinth of
_Rana esculenta_; after Retzius. Enlarged five times.
_ac._ Aquaeductus cochleae.
_av._ Aquaeductus vestibuli.
_cp._ Posterior semicircular canal.
_cra._ Canalis rami anterioris acustici.
_crp._ Canalis rami posterioris acustici.
_fr._ Fenestra rotunda.
_fsc._ Fovea sacculi et cochleae.
_ft._ Trigeminal foramen.
_kn._ Cartilaginous suture.
_ol._ Exoccipital.
_pr._ Prootic.
_ps._ Parasphenoid.
]
The lower half of the supero-external surface (under the _processus
squamosus_) is irregularly concave; immediately under the _proc.
squamosus_ is a shallow groove in the prootic; it is continued on the
exoccipital to the jugular foramen. The remaining part of this surface
consists of a rounded cartilage, and is part of the primordial-cranium
cartilage between the prootic and the exoccipital; posteriorly it has
a small oval aperture, the _foramen ovale_ (_fenestra vestibulare_),
which with the groove is covered by the _columella_. Above the aperture
the cartilage narrows and is continued to the _processus squamosus_.
(2) The *posterior surface* is directly continuous with the
supero-external surface, and is formed by the exoccipital. This surface
lies behind the ridge formed by the posterior semicircular canal, it
is concave, and has two small apertures, separated by a narrow, bony
process, and situated near the jugular foramen; these are the _foramen
rotundum_ (_fr._) and the _aquaeductus cochleae_ (_ac._).
(3) The *anterior surface* is altogether bony and formed by the prootic
(_pr._); the anterior surface of this part of the wall is continuous
with the surface of the cranium, and abuts on to the large trigeminal
foramen (_ft._); below it is continuous with the under surface of the
cranium, below and externally it articulates with the pterygoid.
(4) The *interno-inferior surface* is as a whole hollowed internally,
the upper part being pushed in towards the cranial cavity. It is formed
by the prootic anteriorly, by the exoccipital posteriorly, and is
completed by the cartilage lying between these bones. In the middle
of the upper part of the cartilage is a small oval opening (_av._),
the _ap. aquaeductus vestibuli_: about midway between this aperture
and the jugular foramen is a second opening in the cartilage (_cra._),
the opening of the _canalis rami anterioris (vestibularis) acustici_:
a little behind and above this is the opening of the _canalis rami
posterioris (cochlearis) acustici_, situated in the exoccipital. The
cartilage between the prootic and exoccipital in the lower half of the
interno-inferior surface rests on the parasphenoid.
*b.* The *position* of the *soft parts* in the capsule. The large
outer, rounded part of the cavity contains the saccule and cochlea
(_fovea sacculi et cochleae_) (_fsc._). Above, the cavity is more
irregular, and contains the utricle and the _sinus utriculi superior_.
In front and externally, imbedded in a deep groove, are the anterior
and external ampullae (_fovea ampullae anterioris et amp. externae_);
to these are attached the corresponding semicircular canals. The
anterior semicircular canal opens above and externally into the
canal of the _sinus utriculi superior_; the posterior passes into a
groove for the posterior ampulla (_fovea ampullae posterioris_), and
opens above and internally into the posterior semicircular canal.
The external semicircular canal courses in the _processus squamosus
prootici_, the posterior in the cartilage (Fig. 244 _cp_) between the
prootic and exoccipital, and the anterior in the prootic. The upper
and posterior part of the anterior semicircular canal is imbedded in
cartilage, on which rests the parasphenoid.
*c.* The *perilymphatic space* (Fig. 245 I and II). The membranous
labyrinth does not occupy the whole space enclosed by the hard parts
but is surrounded by the perilymphatic space, which contains the
perilymph. The periosteum and perichondrium form the outer lining of
the space, and are for the most part pigmented:
The perilymphatic space is widest in the lower part of the organ, in
the region of the _fovea sacculi et cochleae_, especially externally
and behind (_per_); in front it is much narrower (_per^1_). A little
higher, in the region of the _tegmentum vasculosum_, the space is
wanting, as the membranous labyrinth is here attached to the periosteum.
Around the utricle and saccule the space is comparatively wide; it is
continued into the bony canals and ampullae. In the semicircular canals
the space is wider on the concave side than on the convex side, the
membranous canals being placed eccentrically, as are also the ampullae,
though to a less extent. The periosteum and perichondrium lining this
cavity form a very delicate, pigmented membrane, formed of numerous
fine elastic fibres which cross each other irregularly; its inner
surface is lined with an incomplete layer of branched protoplasmic
cells with large oval nuclei. From this membrane numerous irregular
bundles of fibres, in the form of trabeculae, pass into the space and
form a rich network, which is attached internally to the outer surface
of the membranous labyrinth and holds it in position. Free nucleated
leucocytes are occasionally met with in the meshes of this network. The
perilymphatic space is prolonged into two subsidiary cavities:--
[Illustration: Fig. 245.
The membranous labyrinth of _Rana esculenta_, within its natural
covering of periosteum; after Retzius.
I. Seen from the side; magnified ten times.
II. The hinder half; magnified ten times. Seen obliquely from above,
and in front, and from the side.
_aa_ Anterior ampulla.
_adf_ Apertura fenestrae ovalis.
_al._ Apertura lagenae.
_ap._ Posterior ampulla.
_apb._ Opening into the pars basilaris.
_ca_ Anterior semicircular canal.
_ce_ External semicircular canal.
_cp_ Posterior semicircular canal.
_dfo_ Ductus fenestrae ovalis.
_dp_ Ductus perilymphaticus.
_dp′_ Saccus perilymphaticus.
_l._ Lagena cochlea.
_ms._ Macula sacculia.
_pb_ Pars basilaris cochleae.
_per_ Wider part of perilymphatic space.
_per^1_ Narrower part of perilymphatic space.
_ra._ Ramus anterior.
_rn._ Ramulus neglectus.
_rp._ Ramus posterior.
_rs._ Ramulus sacculi.
_s._ Saccule.
_sfo_ Saccus fenestrae ovalis.
_spl_ Saccus perilymphaticus.
_tv_ Tegmentum vasculosum.
_u_ Utricle.
]
(1) The *ductus fenestra ovalis* (Retzius), (Fig. 245 _dfo_) commences
as an opening (_adf_) immediately opposite the _foramen ovale_ of the
hard capsule, which leads into a moderately large canal, the structure
under consideration. It passes forwards and outwards through the
_fenestra ovalis_ between the interstapedial and the outer wall of
the capsule into the oval depression (_fossa fenestrae ovalis_), and
rapidly widens to form a short, flattened, blind sac (_saccus fenestrae
ovalis_), which is lodged in the fossa (_sfo_). Its walls are thin and
pigmented.
[Illustration: Fig. 246.
Part of the outer wall of the perilymphatic space: after Retzius.
Vérick’s Syst., Obj. III, Oc. 3.
_per._ Periosteum.
_pg._ Perilymphatic network.
_zn._ Leucocytes.
]
(2) The *ductus perilymphaticus* (Hasse), (Fig. 245 _dp_) passes above
and behind the _ductus fenestrae ovalis_, behind the auditory-nerve,
and near the cochlea, then behind and above the _lagena_ to the
_aquaeductus cochleae_ (Fig. 244 _ac_), where it forms a short, wide
tube, which passes backwards and inwards through this canal to the
_canalis jugularis_; here it lies close to the nerves and forms a short
oval sac (_saccus perilymphaticus_) (_dp′_), which communicates with
the sub-arachnoid space of the cranial cavity by means of a tube from
the neck of the sac. The walls of this structure are thin and formed
of connective-tissue with very few pigment-cells. The other extremity
of the tube passes to the _pars basilaris_ and under the _sinus post.
utri._, between it and the _pars neglecta_; it then courses to the
outer side of the hinder end of the external semicircular canal: the
tube then bends downwards to the outer side of the utricle and saccule
to open into the general perilymphatic space.
*d.* The *membranous labyrinth* (Figs. 247, 248) has the following
parts: the utricle and _sinus superior_, the _recessus utriculi_,
the anterior semicircular canal and anterior ampulla, the external
semicircular canal and external ampulla, the posterior semicircular
canal and the posterior ampulla, the saccule, _ductus endolymphaticus_,
and _saccus endolymphaticus_, the _pars neglecta_, the _lagena
cochleae_, the _pars basilaris cochleae_, and the so-called _tegmentum
vasculosum_.
In addition the following nerve-terminations can be distinguished: (1)
The _macula ac. recessus utriculi_, (2) the three _cristae acusticae
ampullorum_, (3) the _macula ac. sacculi_, (4) the _macula ac.
neglecta_, (5) the _papilla ac. lagenae cochleae_, and (6) the _papilla
ac. basilaris cochleae_.
The auditory nerve divides immediately beyond its origin from the
_medulla oblongata_ to form a _ramus anterior_ and a _ramus posterior_,
which course alongside each other for a short distance, the latter
lying behind and above the former. The _R. anterior_ runs forwards
and outwards under the utricle, giving off the _R. sacculi_, which
runs downwards and outwards; the _R. rec. utriculi_ is then given off
as a number of fibres, which run upwards and forwards; the main nerve
then divides to form the _R. ampullae anterioris_ and the _R. ampullae
externae_, which course together for a short distance and then separate
to reach their respective ampullae. The _R. posterior_ runs backwards
and outwards, gives off the _R. lagenae_, and then divides to form the
_R. neglectus_, running upwards, the _R. basilaris_ running downwards
and backwards, and the _R. ampullae posterioris_ which courses
backwards and outwards.
(1) The *utricle* (_utriculus_) (Figs. 247 and 248 _u_) is irregularly
cylindrical in form: commencing at the _recessus utriculi_ it passes
forwards and outwards; then backwards, inwards, and upwards, to
terminate at the _sinus posterior_, where it is slightly contracted. At
about its middle it is divided into an anterior and a posterior part
by an incomplete, sickle-shaped partition, formed by the posterior
semicircular canal opening obliquely into the utricle, and so causing a
fold in the posterior wall: on the anterior wall there is no fold; the
aperture left in the partition is the _apertura utriculi_.
The posterior part of the utricle receives the _sinus superior_, which
is formed by the junction of the two vertical semicircular canals. The
anterior part of the utricle receives the hinder dilated end of the
external semicircular canal by an opening in its posterior wall, close
to the _apertura utriculi_. In the lower wall or floor is the narrow
opening leading into the _saccule_ (_canalis utriculo-saccularis_):
this opening is placed with its long axis parallel to the long axis
of the utricle, with its broader end posterior, and the narrower end
anterior.
(2) The *recessus utriculi* (Figs. 247 and 248 _rec_). The anterior
end of the utricle widens and curves downwards and outwards to form
the _recessus utriculi_; on its floor is a thin, kidney-shaped plate
or otoliths, resting on the _macula ac. recessus utriculi_ (_mu_),
which receives the _ramulus rec. utriculi_; under it the _ramulus amp.
anterioris_ and the _ramulus amp. externae_ run forwards and close
together to reach their respective ampullae, which are close together
at the antero-external wall of the _recessus utriculi_. The otolith
is a glassy, homogeneous plate, with numerous vacuole-like spaces and
striated borders; it covers the whole of the _macula_.
[Illustration: Fig. 247.]
[Illustration: Fig. 248.
The right membranous labyrinth of _Rana esculenta_; after Retzius.
Magnified 20 times. Fig. 247 seen from the inner side; Fig. 248 seen
from the outer side.
_aa_ Anterior ampulla.
_ae_ External ampulla.
_ap_ Posterior ampulla.
_apn_ Apertura partis neglectae.
_au_ Apertura utriculi.
_ca_ Anterior semicircular canal.
_ce_ External semicircular canal.
_cp_ Posterior semicircular canal.
_cus_ Canalis utriculo-saccularis.
_de_ Ductus endolymphaticus.
_l_ Lagena cochleae.
_mn_ Macula acustica neglecta.
_ms_ Macula acustica sacculi.
_mu_ Macula acustica recessus utriculi.
_pb_ Pars basilaris cochleae.
_pl_ Papilla ac. lagenae.
_ppb_ Papilla ac. basilaris.
_raa_ Ramulus ac. anterioris.
_rae_ Ramulus amp. externae.
_rap_ Ramulus amp. posterioris.
_rb_ Ramulus basilaris.
_rec_ Recessus utriculi.
_rl_ Ramulus lagenae.
_rn_ Ramulus neglectus.
_rs_ Ramulus sacculi.
_s_ Saccule.
_sp_ Sinus utriculi posterior.
_ss_ Sinus utriculi superior.
_tv_ Tegmentum vasculosum.
_u_ Utriculus.
]
(3) The *anterior ampulla* and *semicircular canal* (Figs. 247, 248,
_aa_, _ca_). The anterior ampulla is a rounded, oval vesicle, with
a depressed roof (Fig. 250 I); on the floor is a transverse low
septum, which bears the _crista acustica_ (_cr_) on its free border;
seen from above the _crista acustica_ (Fig. 249 II, _aa_, _cr_) has
concave borders anteriorly and posteriorly; the ends are broad,
rounded, and somewhat raised; and in the middle it presents a small
elevation (Fig. 249 _cr_). On the _crista acustica_ rests the _cupula
terminalis_ (Fig. 250 I, _cu_); this is arched above but of the same
form as the _crista acustica_ below, from which it is separated by an
even slit-like space; the ends are not rounded but hollowed out. The
substance of the _cupula_ is very soft and has parallel striations,
formed of fine fibres and running from above downwards; it separates
very easily from the _crista acustica_. The anterior ampulla is
directed forwards, outwards, and slightly upwards, to open into the
anterior semicircular canal (_canalis m. anterior_) (_ca_), which
curves first upwards and forwards, then backwards, inwards, and
upwards, to open by means of a slightly dilated end into the _sinus
superior utriculi_.
(4) The *external ampulla* and *semicircular canal* (Figs. 247, 248
_ae_, _ce_). The external ampulla lies immediately external to the
anterior ampulla; it also is an oval vesicle, and corresponds with the
anterior ampulla in size and shape, except that the roof is higher.
The roof is directed backwards, the floor forwards; the _septum
transversum_ is low, placed vertically, and bears a triangular,
slightly depressed _crista acustica_: the broader, rounded end of the
_crista_ is directed upwards, the apex downwards. The corresponding
_cupula terminalis_ is relatively high, and is of the same shape as
the _crista_, and is striated. The external ampulla is continuous with
the external semicircular canal (_canalis m. externus_) (_ce_), which
courses outwards and backwards; then backwards, inwards, and slightly
upwards, touches the roof of the posterior ampulla; then curves
forwards and inwards to terminate by a slightly dilated end in the
anterior part of the utriculus.
(5) The *posterior ampulla* and *semicircular canal* (Figs. 247,
248, and 249 _ap_, _cp_). The posterior ampulla commences at the
posterior end of the _sinus post. utriculi_, and is directed outwards
and backwards. It corresponds in all other points with the anterior
ampulla. The posterior semicircular canal (_ce_) (_canalis m. post._),
into which the ampulla opens, curves upwards, inwards, and forwards, to
open into the upper end of the _sinus superior_.
[Illustration: Fig. 249.
The membranous labyrinth of _Rana esculenta_; after Retzius.
I. Part of membranous labyrinth to show relations of the ductus and
saccus endolymphaticus to the cochlear part; magnified.
II. The recessus utriculi and the external ampulla; magnified.
III. To show tegmentum vasculosum, pars basilaris, pars neglecta, etc.
IV. To show relations of the cochlear part to the pars neglecta.
_aa._ Ampulla anterior.
_ae._ Ampulla externa.
_al._ Apertura lagenae.
_ap._ Posterior ampulla.
_apn_ Apertura partis neglectae.
_au._ Apertura utriculi.
_ca._ Canalis m. anterior.
_ce._ External semicircular canal.
_cp._ Posterior semicircular canal.
_cpb._ Opening into pars basilaris.
_cr._ Crista acustica.
_cus._ Canalis utriculo-saccularis.
_de._ Ductus endolymphaticus.
_dp._ Ductus perilymphaticus.
_l._ Lagena cochleae.
_ma._ Macula ac. recessus utriculi.
_mn._ Macula ac. neglecta.
_mp._ Membrana basilaris.
_pb._ Pars basilaris cochleae.
_pe′._ Dark spot on either side of
crest of ampulla.
_pl._ Pars ac. lagenae.
_pn._ Pars neglecta.
_ppb._ Papilla ac. basilaris.
_ra._ Ramus anterior.
_raa._ Ramulus amp. anterioris.
_rae._ Ramulus amp. externa.
_rap._ Ramulus amp. posterioris.
_rb._ Ramulus basilaris.
_rec._ Recussus utriculi.
_rl._ Ramulus lagenae.
_rn._ Ramulus neglectus.
_rp._ Ramus posterior.
_rs._ Ramulus sacculi.
_s._ Saccule.
_sp._ Posterior semicircular canal.
_spl._ Saccus perilymphaticus.
_ss._ Sinus utriculi superior.
_tv._ Tegmentum vasculosum.
_u._ Utricle.
]
(6) The *saccule* (_s_), *ductus endolymphaticus* (_de_), and the
*saccus endolymphaticus* (Figs. 247, 248, 249). The saccule (_s_) is
an oval vesicle, placed under the anterior part of the utricle and
directed outwards and downwards; below it is flattened in a direction
from in front and within, outwards and backwards; above it is wider. In
the inner and anterior surface is the rounded, oval _macula acustica
sacculi_ (Figs. 247 and 248 _ms_), to which is distributed the _R.
sacculi_ (_rs_) from above; a large otolith rests on the macula and
occupies a large portion of the cavity, more especially the lower
portion. The tubular _ductus endolymphaticus_ (_de_) arises by a narrow
oval opening placed in the upper and inner part of the wall, runs
upwards and to the angle between the utricle and the _sinus superior_,
continues in the same direction for a short space, and then curves
inwards to pierce the _apertura aquaeductus vestibuli_, and so reach
the cranial cavity. It then forms a large, thin-walled sac (_saccus
endolymphaticus_), placed between the brain and cranium. The sac is
very vascular and contains crystalline otoliths.
(7) The *pars neglecta* (Figs. 247, 248, 249) was described by Hasse
as the ‘first part of the cochlea’ (‘Anfangstheil der Schnecke’), but
according to Retzius it does not belong to the cochlea. It is placed
above and in front of the _pars basilaris cochleae_, above and a little
behind the _lagena cochleae_, therefore above the upper and posterior
part of the saccule, and under the middle part of the utricle. It is
really a prolongation of the saccule, with which it communicates by
an elongated oval opening placed externally and immediately below the
opening of the _canalis utriculo-saccularis_. It is an oval vesicle,
with its roof intimately united with the lower wall of the utricle;
anteriorly it is broad, posteriorly narrowed. The _macula acustica
neglecta_ is attached to the roof of the vesicle and consists of an
anterior heart-shaped and a posterior semilunar portion united by a
narrow connecting piece (Fig. 250 III and VII). The _ramulus neglectus_
divides into two branches, which supply the two parts of the _macula_.
On the _macula acustica neglecta_ rests the _membrana tectoria_
(Deiters); the membrane is S-shaped, with the anterior end narrow, the
posterior broad (Fig. 250 VII); the borders of the membrane are pierced
by numerous small round holes, the central part has smaller and fewer
perforations, and is finely striated. From the middle of the posterior
part of the upper surface a thicker portion projects downwards into
the cavity of the _pars neglecta_; it has a narrow, deep notch for the
nerve at about its middle; anteriorly it points towards a sickle-shaped
piece, which is curved inwards. Canals run obliquely downwards and
inwards from the apertures on the superior surface. The membrane is
clear, homogeneous, partly finely striated, and corresponds with the
_membrana tectoria_ of the _pars basilaris_, etc.
(8) The *lagena cochleae* (Figs. 247, 248, and 249 _l_) is an oval
swelling of the membranous labyrinth; it lies close to the sacculus
with its broader end forwards, the narrower directed backwards. By a
large rounded opening at the posterior end it communicates with the
saccule by means of a rounded space common to the _lagena cochleae_ and
the _cochlea_. The _papilla acustica lagenae_ (_pl_) is situated in the
posterior wall of the _lagena_; it is elongated, oval, and supplied by
the _ram. lagenae_. The _papilla_ is covered by a plate-like otolith
formed of numerous rounded particles.
[Illustration: Fig. 250.
The membranous labyrinth of _Rana esculenta_; after Retzius.
I. Transverse section of the anterior ampulla. Magnified 50 times.
II. Piece from under surface of the cupula from the anterior ampulla.
Vérick’s Syst., Obj. VIII. Oc. 3.
III. The macula acustica neglecta, seen from below. Vérick’s Syst.,
Obj. IV, Oc. 3.
IV. Part of wall of the anterior ampulla.
V. Part of the cochlea, the pars basilaris cut longitudinally. Vérick’s
Syst., Obj. I, Oc. 3.
VI. Longitudinal section of the pars basilaris. Vérick’s Syst., Obj. I,
Oc. 3.
VII. The macula ac. neglecta, seen from below. Vérick’s Syst., Obj. IV,
Oc. 3.
VIII. The pars basilaris, seen from behind and the outer side. Vérick’s
Syst., Obj. I, Oc. 3.
IX. Transverse section of the external ampulla. Magnified 175 times.
_apb_ Oval opening into sacculo-cochlear space.
_cr_ Crista acustica.
_cu_ Cupula terminalis.
_dp_ Ductus perilymphaticus.
_mb_ Section of thinner wall of pars basilaris.
_mt_ Tectorial membrane.
_mw_ Thickened membranous wall.
_n_ Nerve-fibres.
_pb_ Pars basilaris.
_pe′_ Area of coarsely granular cells.
_ppb_ Papilla acustica basilaris.
_r_ Epithelium on raphe.
_rb_ Ramulus basilaris.
_rn_ Ramulus neglectus.
_tv_ Tegmentum vasculosum.
]
(9) The *pars basilaris cochleae* (Figs. 247, 248, and 249 _pb_)
is placed on the posterior thickened wall of the saccule and lies
above and behind the _lagena_. It forms a small, oval, pocket-like
protuberance, with the long axis directed from in front and above,
backwards and outwards, its opening being directed forwards and
outwards. The short _ram. basilaris_ passes in from above to supply it.
The walls of this dilatation are thick and stiff, with the exception
of a small portion, the _membrana basilaris_ (Hasse) (_mb_), which
closes the opening into a small dilatation on the anterior inner wall.
The _ramulus basilaris_ (_rb_) divides into, at least, two branches,
and passes close to the _membrana basilaris_ (_mb_), where the
elongated and oval _papilla ac. basilaris_ (Fig. 250 _ppb_) is placed.
The _papilla_ is covered by a _membrana tectoria_ (Fig. 250 _mt_),
which is often found separated from the papilla, probably by the action
of the reagents used. The form of this membrane is peculiar but will
easily be understood from the figure (Fig. 250 _mt_). In structure it
is similar to the corresponding structures found in other parts of the
ear.
[Illustration: Fig. 251.
Preparations from the ear of _Rana esculenta_; after Retzius.
I. Part of the membranous wall seen from the surface. Vérick, Obj. VI,
Oc. 3.
II. Transverse section of the membranous wall. Vérick, Obj. III, Oc. 3.
III. Epithelium from the neighbourhood of the macula ac. rec. utriculi.
Vérick, Obj. III, Oc. 3.
IV. Branched cells from the yellow spot on the floor of the anterior
ampulla. Vérick, Obj. III, Oc. 3.
V. Epithelium from the roof of the anterior ampulla. Vérick, Obj. III,
Oc. 3.
_re_ Epithelium of raphe.
_e_ Pavement epithelium.
_pe_ Protoplasmic cells.
_pg_ Perilymphatic tissue.
]
(10) The *tegmentum vasculosum* (Deiters) (Figs. 245, 248, and 250
_tv_) is an oval, shell-shaped dilatation of the membranous labyrinth;
its long axis is directed from above and in front, downwards and
backwards. The walls of the _tegmentum_ are thin and intimately
attached to the periosteum.
*e.* The *minute structure* of the membranous labyrinth (Figs. 251
252).
(1) The *walls* of the membranous labyrinth have the same general
structure throughout: the walls are usually thicker at the
nerve-terminations, in the ampullae, semicircular canals, _pars
neglecta_, and especially the _pars basilaris_; the wall of the
_tegmentum tympani_ are the thinnest. The walls are transparent,
homogeneous, refractive, and, at places, show a faint striation,
which is, as a rule, not due to the presence of fibres; in parts of
the _recessus utriculi_, and in the outer wall of the _saccule_,
especially near the _tegmentum vasculosum_, more or less distinct
fibres can be made out. Sections of the wall show spindle-shaped
cells, with the processes usually arranged parallel to the surfaces;
seen from the surface, the cells are seen to branch in all directions
(Fig. 251 I, II). In the thinner parts of the walls the cells are few
or altogether absent. The outer surface of the membranous labyrinth is
uneven, in consequence of the attachment of the perilymphatic network.
Blood-vessels are also attached to the outer surface, and pierce the
wall, especially near the nerve-terminations.
The whole of the inner surface is lined with a layer of polygonal,
tesselated epithelium-cells. The size and height of the epithelium
varies in different parts. On the outer wall of the saccule the
cells are large, but on the inner wall small; they are also large in
the semicircular canals, except on a small raphe on the inner and
outer side, where they are smaller but higher (Fig. 251 _re_); in
the ampullae the cells are large, except on the roof. In the utricle
and _sinus superior_ they are also moderately large. In addition to
the places mentioned, a smaller epithelium is found on the floors
of the ampullae, in the _recessus utriculi_, and near all the
nerve-terminations and on the sides of the ampullar septa. Surrounding
the nerve-terminations of the _macula rec. utriculi_, _macula
sacculi_, and _papilla lagenae_ are found narrow, branched, yellowish
cells (Fig. 251 _pe_) with spindle-shaped nuclei. Cells of a third
kind, first described by Deiters, Hasse, and Kuhn, in the _tegmentum
vasculosum_, and in the ampullae by Hasse and Kuhn, are also found
in the utricle. They contain a yellowish pigment, and are collected
into two sharply differentiated groups in each ampulla (Hasse has one
placed before and one behind the septum on the floor). The cells are
cylindrical, the upper parts striated, the lower narrower, and the
bases again widened to a polygonal, more homogeneous plate, which is
fixed to the wall. On the _tegmentum vasculosum_ the corresponding
cells are not so high.
(2) The *nerve-terminations*. The larger branches of the auditory
nerve contain medullated fibres of various dimensions and bipolar,
spindle-shaped ganglion-cells. The nerves pierce the walls obliquely
or vertically, and retain their medullary sheaths until near their
final distribution. On each of the nerve-terminations is found
nerve-epithelium, which varies in height in different parts. In the
_crista acustica_ it measures 0.075 mm. in height in the middle part,
0.06 mm. at the sides; on the _macula rec. utriculi_ 0.09 mm., on the
_macula sacculi_ 0.075 mm., on the _papilla lagenae_ 0.06 mm., on the
_papilla part. basil._ 0.045 mm., on the _macula neglecta_ 0.075 mm.
The epithelium is of two kinds, hair-cells and sustentacular cells.
[Illustration: Fig. 252.
The nerve-terminations in the membranous labyrinth of _Rana esculenta_;
after Retzius.
I. Vertical section through the crista acustica of the anterior
ampulla. Vérick’s Syst., Obj. III, Oc. 3.
II. Vertical section through the macula ac. recessus utriculi. Vérick’s
Syst., Obj. VIII, Oc. 3.
III. Three isolated hair-cells from the crista ac. of the anterior
ampulla. Vérick’s Syst., Obj. VIII, Oc. 3.
IV. Two isolated sustentacular cells from the crista ac. of the
anterior ampulla. Vérick’s Syst., Obj. VIII, Oc. 3.
_cr_ Crista acustica.
_cu_ Cupula terminalis.
_fz_ Sustentacular cells.
_h_ Hairs of hair-cells.
_hz_ Hair-cells.
_n_ Nerve-fibres.
]
α. The hair-cells (Fig. 252 _hz_) have, on the whole, elongated,
flask-like forms, but are not all of the same length (0.024–0.04 mm.).
The free ends of the cells are rounded, flattened, and yellowish,
and each bears a stiff cilium, which is fixed by a broad base to the
cell, and thins out towards its free end: the cilia vary in length; in
the ampullae their greatest length is 0.13 mm., on the _macula rec.
utriculi_ 0.011 mm., and on the _papilla lagenae_ 0.017 mm. The cells
are granular, possess rounded oval nuclei, and are fixed by a fine,
narrow process (Fig. 252 _hz_), though they usually seem to be rounded
off without possessing a process.
β. The sustentacular cells. Under the hair-cells is a finely granular
substance, possessing numerous rounded oval nuclei, which are placed in
superimposed rows (Fig. 252 _fz_), the deepest row being placed close
together and immediately on the membranous wall. After proper treatment
and isolation these nuclei are seen to belong to narrow, elongated
cells, which rest by a slightly widened base on the wall, and are
continued upwards between the hair-cells to reach the surface of the
epithelium, where their upper processes are again slightly widened.
γ. The nerve-fibres (Fig. 252 _n_) lose their medullary coats, ascend
towards the epithelium, and frequently divide to form two unequal
branches, which ascend to the level of the hair-cells, and curve so
as to course horizontally as extremely fine varicose fibrillae; these
frequently form a network, of which the exact method of termination
has not been made out. In some cases a fine fibril may be traced to
the base of a hair-cell, but a direct continuation of the one into the
other has not yet been traced.
VI. THE EYE.
(Re-written by the translator.)
The organ of sight, the eyeball (_bulbus oculi_), together with its
appendages (_tutamina oculi_), will be described in this chapter.
A. The Eye is flattened on the outer surface, more convex on the inner
or deeper surface. Its principal axis is directed from behind, forwards
and outwards.
The outer transparent portion of the eyeball is the cornea, which forms
the outer boundary of the anterior chamber. The larger, white, opaque,
and inner portion is the sclerotic coat, which, together with two
deeper tunics, the choroid coat and the retina, enclose the posterior
chamber of the eye. The pigmented ring placed behind the cornea is
the iris, and the aperture it encloses the pupil. The lens is placed
immediately behind the iris. On the inner side the optic nerve pierces
the sclerotic to enter the eyeball.
*a.* The *sclerotic coat* (_sclerotica s. sclera_) forms about
three-fourths of the surface of the eyeball; posteriorly it is pierced
by the optic nerve at a point (_porus opticus_) nearer the temporal
side than the nasal. The sclerotic coat consists of fibrous tissue
externally, with a layer of hyaline cartilage internally (Helfreich).
The fibrous layer is formed of bundles of parallel fibres, which cross
each other, chiefly at right angles (Hoffmann). The cartilaginous layer
ends just behind the line of insertion of the extrinsic muscle of
the eye, and is thickest at the point of entrance of the optic nerve
(Helfreich).
[Illustration: Fig. 253.
Endothelium from the inner surface of the sclerotic coat; after
Hoffmann.
]
The sclerotic coat is rich in nerve-fibres, which form a close network;
the fibres, however, do not unite but form the meshes of the network by
simply crossing each at acute angles.
The deeper surface of the sclerotic coat is lined with a layer of large
endothelial cells (Hoffmann), (Fig. 253), which form the outer wall of
the capsule of Tenon.
*b.* The *cornea* and the *anterior chamber*. The cornea forms about
one-fourth of the surface of the eyeball and is directly continuous
with the sclerotic. In it five layers can be distinguished: a layer of
stratified epithelium or conjunctiva, an anterior hyaline membrane, the
true corneal substance, a posterior hyaline membrane, and a layer of
endothelium.
(1) The *corneal epithelium* is a layer of stratified epithelium
covering the superficial surface of the cornea. The superficial layer
forms a beautiful mosaic of polygonal cells; the middle layers are
polygonal in all sections, while the deepest layer is more or less
columnar. Except in the most superficial layer, all the cells have
serrated surfaces. Smaller cells possessing each two nuclei are also
found between the columnar cells, and are evidently cells in process of
division; according to Waldeyer, cell-proliferation may also take place
in the middle layers.
The basal or deeper portions of the columnar cells possess a clear
border, which reminds one of the hyaline border found on the free
border of columnar epithelium in other parts. The cells are here so
closely applied to one another that these borders have the appearance
of a continuous, highly refracting membrane (Rollett); according to
Henle, the border consists of a network of very fine processes from the
cells above.
(2) The *true corneal substance*, and (3) the *anterior hyaline
membrane*. The corneal substance consists of flat bundles of fibres
arranged in laminae, with cement-substance and connective-tissue
corpuscles interposed. The fibrils are extremely fine (0·0001 mm.,
Engelmann), and bound together into bundles by cement-substance. The
bundles of the laminae are arranged at various angles, though many are
placed at right angles to each other (Waldeyer).
Between the laminae are flattened spaces, which seen in section are
spindle-shaped. By proper treatment they are seen to be irregular,
branched spaces, which communicate by fine canals and form part of the
Recklinghausen-canals or lymph-system. These spaces contain branched,
connective-tissue corpuscles (Toynbee), and a colourless fluid.
The corpuscles (Fig. 254 _e_) do not fill the spaces which they occupy.
They possess large nuclei, surrounded by granular protoplasm.
[Illustration: Fig. 254.
Preparation from cornea of _Rana esculenta_; after Klein, Hartnack’s
Syst., Obj. VII, Oc. 3.
_a_ Nerve of first order.
_b_ Nerve of second order.
_c_ Nerve of third order.
_d_ Nerve of fourth order.
_e_ Corneal corpuscles.
]
The canals by which these spaces communicate (‘Saftcanälchen’ of
Recklinghausen) lie, in general, parallel to the surfaces of the
cornea, and communicate by joining at acute angles or by short
transverse branches. According to Lavdowsky, these canals have a
distinct lining membrane.
The anterior hyaline[87] layer (Bowman’s or Reichert’s lamella) is not
so well seen in the frog as in some higher animals; it is simply a
portion of the corneal substance, of somewhat denser structure than the
rest, into which it passes by a gradual transition.
[Footnote 87: Tamanscheff and Schweigger-Seidel consider the anterior
and posterior hyaline membranes to be composed of fine fibrils.]
(4) The *posterior hyaline membrane* (Descemet’s membrane) is a highly
elastic, very transparent layer, placed behind the true corneal
substance; in the frog some few bundles of fibres belonging to the
true corneal substance appear to pass into the posterior hyaline
layer, although they cannot be traced further through its substance.
The structure of the membrane is, in consequence of its transparency,
unknown, though the above observation seems to point to a fibrillar
origin.
(5) The *corneal endothelium* is a single layer of polygonal cells of
0·02 mm. diameter. The cells possess the power of altering their shape
when stimulated (Klebs).
[Illustration: Fig. 255.
Preparation from cornea of _Rana esculenta_; after Klein. Hartnack’s
Syst., Obj. X immers., Oc. 3.
_a_ Endothelial cells.
_b_ Nuclei of endothelial cells.
_c_ Nerves of third order in the tissue of the cornea propria.
_d_ Nerves of the fourth order.
]
(6) The *nerves* of the cornea are derived from the _ramus ophthalmica
trigemini_; they pierce the sclerotic coat in front of the sclerotic
cartilage and then course towards the cornea, at the margin of which
they form a coarse network of medullated fibres. From this about thirty
nerves pass towards the cornea, which they enter, and then very quickly
lose the main part of their medullary sheaths. According to Wolff, a
portion of the nerves retain their medullary sheaths, or in some cases
appear to regain it after having lost it.
The nerves passing from the plexus (nerves of the first order, Klein)
give off smaller branches, which for a short distance have a serpentine
or rectilinear course. By a few anastomoses they form a loose plexus
(nerves of the second order, Klein). After a longer or shorter course
they give off numerous lateral fibres, or terminate in several such
fibres arising at one point (nerves of the third order, Klein). These
are distinguished by their size, varying only within small limits,
and by the possession of more or less regularly placed varicosities;
the clearer portions are longitudinally striated as though made up of
fibrillae; they have a nearly rectilinear course, and, after a longer
or shorter course, turn into a direction which is at right angles to
the former one; lastly, they remain for long distances unbranched.
These nerves are connected one with another by cross fibres running at
right angles to them, and in this way a rectangular trellis-work is
formed.
The fibrils (nerves of the fourth order) given off by these nerves
form networks around the connective-tissue corpuscles, but no direct
connection between nerve and corpuscle has been traced; they always
appear to lie on that surface of the corneal corpuscle which is
directed towards the superficial surface of the cornea (Klein). In
the endothelium covering the membrane of Descemet these fibrils can
be traced coursing along the margins of the cells (Fig. 255 _d_), and
sometimes undergoing dichotomous division (Klein).
Almost all observers have described these fibrils as possessing
varicosities; Hulke, and more recently Wolf, however, deny their
presence. Lavdowsky traces nerve-fibrils to the nuclei of the
connective-tissue corpuscles.
(7) The *anterior chamber* is the space between the cornea and the
iris, and is filled with a watery fluid, the aqueous humour. At
the circumference of the chamber are a number of spaces (spaces of
Fontana), formed by interruptions in the tissue between the posterior
surface of the cornea and the iris; the result is that bands or
trabeculae (_ligamentum pectinatum iridis_) pass from the one structure
to the other, and between these are the spaces of Fontana.
According to Angelucci these trabeculae are of three kinds: trabeculae
passing from the cornea to the iris, formed of connective-tissue;
trabeculae from the cornea to the ciliary processes, which contain
elastic tissue; trabeculae from the interstitial connective-tissue of
the ciliary muscle to the cornea, and formed almost entirely of elastic
tissue.
At the junction of the cornea and sclerotic, and just in front of
the spaces of Fontana, is a larger and similar space, which may be
traced round the whole circumference of the cornea; this, the canal of
Schlemm (_Sinus circularis iridis_), is held to be a venous plexus by
some observers (Angelucci, and others), according to others it is a
lymphatic space in connection with the anterior chamber (Schwalbe, and
others). It is certain that the vessels can be very easily injected
from the anterior chamber, although a direct communication has not yet
been seen.
*c.* The *choroid coat* and the *iris* (_tunica choroidea et iris_,
_tunica vasculosa_).
[Illustration: Fig. 256.
The vessels of the choroid and iris; after Hans Virchow.
I. The two roots of the V. bulbi superior. Magnified 10 times.
II. Vessels of the iris. Magnified 9 times.
III. Origin of the ventral vein.
IV. Schema of the choroid vessels; seen from the proximal pole.
V. Transverse section through the choroid at the equator.
VI. The origin of the choroid arteries from the ophthalmic artery.
The greater part of the sclerotic has been removed.
Left eye twice natural size.
A. From the proximal pole. B. From the temporal side.
VII. Part of a choroidal artery attached to the choriocapillaris.
Magnified 10 times.
VIII. A portion of the choriocapillaris, more highly magnified.
IX. The V. ophthalmica and V. bulbi superior on the sclerotic of
the right eye.
A. Seen from proximal pole. B. Seen from above.
Twice natural size.
_A_ Art. ophthalmica.
_A′_ Art. choroidea.
_An_ R. nasalis of the ophthalmic artery.
_At_ R. temporalis of the ophthalmic artery.
_ch_ Area of membrana choriocapillaris.
_Le_ Outer pigmented layer of choroid.
_Li_ Inner pigmented layer of choroid.
_N_ Optic nerve.
_r_ Vasa recta.
_r′_ Transitional part between choriocapillaris and ventral whorl.
_r″_ Transitional part between choriocapillaris and upper whorl.
_R_ Branches of the circulus iridis major.
_Rd_ Distal root of vein of under surface of eye.
_Rd′_ Distal root of nasal vein.
_Rd″_ Proximal root of nasal vein.
_Rp′_ Nasal root of ventral vein.
_Rp″_ Temporal root of ventral vein.
_Vbs_ V. bulbi superior.
_Vh_ V. hyaloidea.
_Vo_ V. ophthalmica.
_Vp._ Proximal root of vein of under surface of eye.
_Vs′_ Nasal root of V. bulbi superior.
_Vs″_ Temporal root of V. bulbi superior.
]
*1.* The *choroid coat* lines the deeper surface of the sclerotic coat,
but is also prolonged under the cornea to form the iris. The choroid
is firmly attached to the sclerotic in two positions, at the point
of entrance of the optic nerve, and at the line of junction of the
sclerotic and the cornea. Its external surface is closely applied to
the deeper surface of the sclerotic, from which it is only separated
by a very narrow serous cavity (supra-choroidal space), and to which
it is attached by numerous vessels and nerves. The deep surface of the
choroid is covered by the retina, to which it is closely attached,
except at the ora serrata, the attachment being especially intimate at
the _processus ciliares_.
The choroid coat consists of a fibrous layer containing corpuscles
and traversed by a very rich vascular anastomosis. The corpuscles
of this layer are deeply pigmented, in some cases to such an extent
that the oval nucleus cannot be seen; the fibrous tissue is also
pigmented, and has consequently a brownish tinge. That portion of the
layer immediately below the sclerotic is termed the _lamina fusca_
or _suprachoroidea_, the vessels on the deeper surface forming the
_membrana choriocapillaris_. This again is lined on its deeper surface
by a hyaline membrane.
α. The arteries (Fig. 256 VI; VII, VIII) supplying this coat are two
branches of the _arteria ophthalmica_; these form a capillary network
(Fig. 256 VII) resembling the corresponding structure found in mammals.
The meshes have approximately the same size, while the capillaries
themselves vary considerably in size. This network is, however, only
complete on the nasal, temporal, and proximal part of the upper
surfaces. Towards the _corpus ciliare_ the meshes become wider and
elongated; the capillaries then unite at acute angles parallel with
the longitudinal axis of the eye. The network (_choriocapillaris_)
exists in a simple layer within the two arteries which form it, and
superficial to the veins (Virchow).
β. The veins of the choroid (Fig. 256 III, IV, IX) are (1) a vein which
unites at the lowest point of the equator of the eye with the _V.
hyaloidea_ to form (2) the _V. ophthalmica_, two small branches of the
_V. bulbi superior_, which unite outside the sclerotic, and (3) the
_vasa recta_.
(1) The larger vein arises from the greater part of the under surface
of the eye; it gives off branches to each side, which radiate to form
a ‘whorl’ or star-shaped capillary anastomosis (Fig. 256 III), the two
halves of which have no connection. A proximal and a distal root can be
distinguished in the anastomosis; the distal lies towards the _corpus
ciliare_, and occupies exactly one-fourth of the circumference of the
choroid at its junction with the _corpus ciliare_.
(2) The two branches of the _V. bulbi superior_ lie alongside the
_corpus ciliare_ on the upper surface, and each occupies one-fourth of
the circumference; they form a similar though simpler figure (Fig. 256
I) to the foregoing, each forming one half.
(3) The _vasa recta_ are numerous parallel vessels which arise in the
iris, and coursing centrally empty themselves into the branches of the
_V. bulbi superior_ on the superior surface, and into the branches of
the venous capillaries on the inferior surface.
*2.* The *iris* is covered anteriorly by a layer of endothelium,
continuous with that covering the posterior surface of the cornea, and
of similar character. The border of the pupil (_margo pupillaris_)
is of a golden colour, outside this bright ring to its outer margin
(_margo ciliaris_) the iris is black; the golden colour is due to the
presence of cells containing a pale yellow pigment; the nuclei of
these cells are round and granular; the cells themselves have rounded
outlines (Hoffmann). The black portion of the iris contains more
irregular, spindle-shaped cells, with round nuclei, which are hidden by
a dense mass of pigment-granules (Iwanoff and Hoffmann).
The true substance of the iris consists of muscle, nerves,
blood-vessels, and a connective-tissue stroma, but on the posterior
surface is another layer of black, pigmented cells, and this is again
covered with a hyaline membrane, in which, however, a fibrous structure
may be made out (Koganeï).
The muscle-fibres are long, spindle-cells, which are abruptly swollen
in the middle, where the nuclei are situated; the nucleus is oval,
0.009–0.0012 mm. in length, 0.0025 mm. broad, and occupies nearly the
whole of the swollen part of the cell (Hoffmann, Grünhagen).
According to Koganeï the iris possesses a _M. constrictor iridis_
(_l. c._ Berlin Sitzungsber.), but no _M. dilatator iridis_; in a
former publication (_l. c._ Arch. mik. Anat.) he was unable to find
any muscular fibre, and holds the muscle-fibres of Grünhagen to be
connective-tissue elements.
The stroma consists of delicate connective-tissue fibrils, enclosing a
very large number of pigmented, branched cells.
α. The *arteries* of the iris (Fig. 256 II) arise from an arch (see
Vessels of Eye) formed by the _A. ophthalmica_ in the _corpus ciliare_.
It commences between the ventral and temporal surfaces by two branches:
one courses along the temporal border, the other along the nasal,
to meet each other on the nasal surface; the former courses through
one-third, the latter embraces two-thirds of the circumference at the
iris.
The temporal artery courses along the ciliary border during the first
third of its course, it then gradually approaches the border of the
pupil; the nasal artery runs at once towards the pupil. On the nasal
border of the pupil they anastomose by their branches, and so form a
_circulus iridis major_.
Except near their termination, no small vessels arise from this
arterial circle; in Fig. 256 II, for example, only five larger branches
are given off, three from the temporal side and two from the nasal.
The five large branches run towards the circumferential border of the
iris and break up into numerous vessels, which form a very irregular
and open network. From this network arise the _vasa recta_ already
described.
*d.* The *lens* is almost spherical, and is composed of cellular
elements enclosed in a capsule (_capsula lentis_).
The capsule is a homogeneous, transparent, structureless, and highly
elastic membrane. The deeper surface of the anterior capsule is lined
with a simple layer of regular nucleated six-sided epithelial cells.
The lens itself consists of long, flat fibres; seen from the surface
these are broad, narrow edge-wise, and in section six-sided prisms.
Those lying parallel to the anterior and posterior surfaces are broad
and thicker, those towards the border are narrower. These cells are
striated, both longitudinally and transversely (Arnold). The cells near
the margin, however, have no transverse striation (Hoffmann). The cells
of the central parts form a much closer and firmer structure than those
at the periphery (Arnold). The peripheral cells are nucleated, and
sometimes even possess two nuclei to one cell; the central cells have
no nuclei (Arnold).
The cells are held together by a cement-substance and by their serrated
surfaces; the serrations are the cause of the transverse striations.
The fibres of the lens have a simple arrangement: commencing at the
middle point or pole of one surface they pass over the equator to the
opposite pole; consequently the long borders of adjacent cells are in
juxtaposition, and their pointed extremities meet at points in the axis
of the lens (Hoffmann).
Ritter has described short, nucleated cells in the centre of the lens;
these are held by Babuchin to be cells which have been arrested in
their development.
[Illustration: Fig. 257.
Fibres from the lens of the frog; after Hoffmann. Magnified 700 times.
]
*e.* The *retina* is the innermost coat of the eye; in the recent
state it is pale, soft, and smooth. The structures composing it are
arranged in ten layers; from the deeper surface towards the choroid
these are: the internal limiting membrane, the optic-fibre layer, the
ganglion layer, the inner molecular layer, the inner nuclear layer, the
outer molecular layer, the outer nuclear layer, the external limiting
membrane, the layer of rods and cones, and the pigment layer.
These layers are held together by connective-tissue elements.
(1) The *internal limiting membrane* (_Membrana limitans interna_) will
be described together with the connective-tissue elements (10).
(2) The *optic-fibre layer* is formed by the fibres of the optic
nerve. The nerve-fibres in their course towards the eye are possessed
of medullary sheaths, but on piercing the sclerotic these sheaths are
lost. The fibres are now pale, non-medullated, and of very varying
thickness. In the mass of fibres nothing can be seen except an
extremely fine fibrillation and very fine varicosities; the latter,
however, appear to be artificial productions (Hoffmann). This layer
of fibres extends over the inner surface of the retina, and gradually
thins from the point of entrance of the optic nerve to the limits of
the retina.
(3) The *ganglion-layer* lies immediately without the nerve-fibre layer
(Fig. 258 _b_). The ganglion-cells are small and usually pear-shaped.
The cells possess large nuclei, round which is a thin layer of very
granular protoplasm. The cells have inner and outer processes; the
inner pass into the nerve-fibre layer, the outer into the inner
molecular layer in more or less radiating directions. Manz claims to
have traced a direct connection between the inner processes and the
fibres of the nerve-fibre layer.
Each ganglion-cell, whatever its shape or size, has only one inner
process, which is easily distinguished from the outer process by its
being more glistening, by the possession of varicosities, and because
this process never branches.
The outer processes are single (Schwalbe) or rarely double (Hoffmann),
and have as a rule a direction at right angles to the inner processes.
Each outer process is finely granular, which suggests rather a
prolongation of the cell-substance than a true process. Frequently they
are branched, sometimes forming two equal sized processes, which give
off finer twigs; at other times they appear to pass through the whole
of the inner molecular layer without undergoing division (Schwalbe).
The processes do not inosculate (Santi Sirena).
[Illustration: Fig. 258.
Vertical section through retina of frog; after Hoffmann. Magnified 500
times.
_a_ Internal limiting membrane.
_b_ Ganglion-cell layer.
_c_ Internal molecular layer.
_d_ Internal nuclear layer.
_e_ External molecular layer.
_f_ External nuclear layer.
_g_ Layer of rods and cones.
_h_ Pigmented epithelium layer.
1. Inner segments of rods and cones.
2. Outer segments of rods and cones.
3. Outer transparent segments of pigmented epithelium.
]
(4) The *inner molecular layer* (Fig. 258 _c_) is 0·07–0·08 mm. thick
(Hoffmann), and consists of a finely granular mass together with the
outer processes of the ganglion-layer, and connective-tissue elements.
The granular matter consists of an extremely fine network or reticulum,
through which numerous fine fibres course (Schultze, Kölliker, Manz,
Heinemann, and others); according to Schultze the supposed molecules
or granules of others (Henle, Merkel, and Retzius) are simply the
fine meshes of this reticulum. The branched, outer processes of the
ganglion-cells form a rich anastomosis in this layer.
(5) The *inner nuclear layer* (Fig. 258 _d_) contains parts of two
kinds of cellular elements; these are radial nerve-fibres with
large nuclei, and connective-tissue elements (see below, par. 10).
The nerve-fibres are easily distinguished by their spindle-shaped
varicosities; both cellular elements possess large oval nuclei. The
bodies of the cells surrounding the nerve nuclei are almost filled by
the nuclei, which have sharply-defined, rounded nucleoli. The fibres
to which these cells are attached may be distinguished as inner and
outer processes; the inner process is fine, irregularly varicose, and
unbranched; the outer process is thicker, finely granular, and is not
varicose (Schwalbe). At the margin of the outer molecular layer the
outer processes divide, usually into two branches, and at an acute
angle to each other, though sometimes at a right angle. The further
course of these branches in the outer molecular layer is unknown.
(6) The *outer molecular layer* (Fig. 258 _e_) corresponds in general
with the inner molecular layer as regards its structure; it is,
however, much thinner.
(7 and 9) The *outer nuclear layer* and the *layer of rods and cones*
(Figs. 258 _f_, _g_, 259). The rods and cones are intimately connected
with the elements of the outer nuclear layer, hence the two layers are
best described together.
The rods (_bacilli_) have two parts or limbs, an outer and an inner,
which differ in structure, and in chemical and physical characters. The
outer part is highly refractive, the inner more homogeneous and less
refractive, the two parts being sharply differentiated from one another.
The outer part is also weakly doubly refracting, the inner has no
trace of this property. The rods are 0·05–0·06 mm. in length, of
which 0·035–0·04 mm. belongs to the inner limb. The outer end of the
outer limb is more or less rounded; the whole has a longitudinal
striation (Schultze), due to its being composed of rounded fibrils,
about twenty-four to each rod (Hensen). The fibrils are sharply
differentiated from each other and have a slightly spiral course; when
seen in transverse section these outer limbs do not appear to be round
(Schultze), although others hold them to be perfectly rounded (Hoffmann
and others), and that the loss of the cylindrical form is due to the
methods of treatment. According to Merkel the longitudinal striation
is caused by a canalisation of the outer limb, which according to him
encloses the processes of the pigmented epithelial layer; he is also
of opinion that the spiral appearance is an artificial product. In the
latter opinion he is probably wrong, as perfectly fresh rods examined
in aqueous humour show the same spiral appearance (Hoffmann): against
the canalisation view others observe that the longitudinal striation is
most distinct near the inner limit of the outer limb, and that it is
impossible to conceive that the processes of the pigment-cells should
terminate with such extremely regular ends (Hoffmann).
[Illustration: Fig. 259.
Various preparations from the eye of the frog: chiefly from the retina.
1. Rod from retina in aqueous humour, showing spiral striation.
2. Three rods and one cone after treatment with osmic acid.
3, 4, 5. Rods examined in recent state.
6, 7. Inner segments of two rods after treatment with osmic acid.
_a_ Outer limb.
_b_ Inner limb.
_c_ Lenticular body.
_d_ Nucleus of outer nuclear layer.
_e_ External limiting membrane.
8. Nuclear body from inner nuclear layer.
9. Twin-cone.
10. Sustentacular fibre of retina.
11. Surface view of pigmented epithelium of retina.
12, 13. Isolated pigmented cells of retina.
14. Four pigmented cells, rods and cones, external limiting
membrane, and part of outer nuclear layer attached.
15. Two pigmented cells; each showing three attached rods.
16. Muscle-fibre from the iris.
Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9 are magnified 500 times. Fig. 10, 300
times. Fig. 16, 400 times. All are copied from Hoffmann’s figures.
Figs. 12, 13, 14, 15 are copied from Morano’s figures; Hartnack, Oc.
II, Obj. 9.
]
In the central part of the inner end of the outer limb is seen a dark
point when the structures are examined in transverse section (Ritter,
Manz, Schiess, Schultze, and others). The cause of this is not clearly
understood; some hold it to be a fibre (Ritter’s fibres), others hold
it to be an artificial product (Hensen).
After treatment with certain reagents the outer limbs show a transverse
striation, which is probably produced by the action of these reagents
on the sheath of the outer limbs; that a sheath is present is proved by
its possession of a different refractive index (Zenker, Schultze) to
the rest of the outer limb, and this transverse striation is not seen
until the whole organ has undergone considerable _post-mortem_ changes
(Hoffmann). Should this change be allowed to proceed a stage further,
the outer limbs of the rods split transversely and form small discs
from 0·0005–0·00055 mm. thick; this takes place in the outer limb only.
The inner segments of the rods (Figs. 258, 259) are short
(0·020–0·022 mm.) and of the same thickness as the outer limbs. When
perfectly fresh they appear homogeneous; very quickly changes commence,
which are probably due to coagulation. A plano-convex figure (Fig. 259)
is then seen at the outer portion of the segment (lens-shaped figure
of Schultze); with staining reagents it gives the same reactions as
the outer segment of the rods. The rest of this segment forms a short
cylinder, which probably has no distinct sheath (Hoffmann, Merkel);
some observers are inclined to think that a sheath exists (Landolt,
Schwalbe).
The outer segments of the rods are of two chief sizes (Schwalbe). Those
of the one kind are large; the second variety occurs less frequently,
and the segments are shorter, measuring only 0·002 to 0·0025 mm. The
inner segment is a long, thread-like process, except where it is
swollen to enclose the lens-shaped body.
The rods are much more numerous than the cones, except at one small
spot (_macula lutea_) on the posterior surface of the retina, where
only cones are found (Krause).
The cones (_coni_) have each two segments like the rods (Figs. 258,
259). The outer segments are short (4–5 µ), they are slightly conical
and terminate externally in a blunt point; they possess a longitudinal
striation (Schultze), and very easily break up transversely into small
discs, which, however, do not separate so completely as in the case of
the rods, in consequence of the presence of a sheath continuous with a
sheath on the inner segment.
The inner segments (Figs. 258, 259) have convex sides and measure
12–14 µ; like the corresponding parts of the rods they possess
lens-shaped bodies at their junction with the outer segments, but the
bodies differ in shape, being bi-convex or rather oval in form. The
inner segments are enclosed in a delicate sheath continuous with that
of the outer segments.
In some cases two cones are united to form a twin-cone; in such cases
the one is always larger than the other (Fig. 259 9), and has
several peculiarities which distinguish it from the smaller.
The smaller or secondary member of a twin-cone is longer, and possesses
a lens-shaped body which is plano-convex. The larger or principal
member of a twin-cone is shorter, has a plano-convex body, but also an
oval, homogeneous, glistening body, which is directly attached to the
plano-convex body. The shape of the two members is also different.