Vertebrata.
The most complicated condition in which matter exists is where, under
the influence of life, it forms bodies with a structure of tubes and
cavities in which fluids are incessantly in motion, and producing
continuous changes. These have been rightly designated “organised
bodies,” because of the various organs they contain. The two principal
classes into which organised bodies have been divided are recognised as
vegetable and animal. It was Bichat who taught that our animal life is
double, while our organic life is single. In organic life, to stop is
to die; and the life we have in common with vegetables never sleeps,
and if the circulation of the fluids within the animal body ceases for
a few seconds, it ceases for ever. In the vertebrate body, however, the
combination of organs attains to the highest development, in striking
contrast with that of the class we have previously considered, the
Invertebrata, the animal kingdom being divided into Vertebrates and
Invertebrates.
The Vertebrata are distinguished from all other animals by the
circumstance that a transverse and a vertical section of the body
exhibits two cavities completely separated from one another by a
partition. A still more characteristic feature separates the one from
the other; it is the specialisation of the chief nervous centres, and
their peculiar relation to the other systems of the body.
The dorsal cavity of the body contains the cerebro-spinal nervous
system, the ventral, the alimentary canal, the heart, and usually a
double chain of ganglia; these pass under the name of the sympathetic
system. It is very probable that this sympathetic nervous system
represents, wholly or partially, the principal nervous system of
the Annulosa and Mollusca. In any case, the central parts of the
cerebro-spinal nervous system--_i.e._, the brain and the spinal
cord--would appear to be unrepresented among invertebrate animals.
Likewise, in the partition between the cerebro-spinal and visceral
tubes, certain structures which are not represented in Invertebrates
are contained. During the embryonic condition of all Vertebrates,
the centre of the partition is occupied by an elongated cellular
cylindrical mass, the notochord, or chorda dorsalis. This structure
persists throughout the life in some Vertebrata, but in most it
is more or less completely replaced by a jointed, partly fibrous,
cartilaginous, and bony vertical column. All vertebrate animals have
a complete vascular system. In the thorax and abdomen, in place of a
single perivisceral cavity, in communication with the vascular system,
and serving as a blood-sinus, there are one or more serous sacs. These
invest the principal viscera, and may or may not communicate with
the exterior, recalling in the latter case the atrial cavities of
the Mollusca. In all Vertebrata, except Amphioxus, there is a single
valvular heart, and all possess a hepatic portal system, the blood of
the alimentary canal never being wholly returned directly to the heart
by the ordinary veins, but being more or less completely collected into
a trunk (the portal vein), which ramifies through and supplies the
liver.
With reference to one other point of importance, the development of the
ova of Vertebrates, these have the same primary composition as those of
other animals, consisting of a germinal vesicle containing one or more
germinal nuclei, and included within a vitellus. But as this forms a
part of general anatomy, and as my object is simply the investigation
of the fundamental and microscopical structure of animal organisms,
I shall not further pursue the morphological part of the subject,
especially as so many excellent text-books are within reach of the
student who desires to fully acquaint himself with precise information.
Notwithstanding, then, the apparent diversity in the structure of
the vertebrate and the invertebrate and the various tissues of which
animals and vegetables are constituted, microscopical research has
satisfactorily demonstrated that all textures have their origin in
cells; in fact, when the formative process is complete, the animal
cell is seen to consist of the same parts and almost the same chemical
constituents as the typical cell of the plant--namely, a definite
cell-wall enclosing cell contents, of which the nature may be diverse,
but the cell nucleus is precisely the same and is the actual seat
and origin of all formative activity. The cell and nucleus grow by
assimilation or intersusception, that is, by inflowing of nutrition
among all parts, the new replacing the old, yet maintaining its
original structure and composition. That which was once thought special
to animals is now found to be common to both plants and animals: they
are found to be alike fundamentally in internal structure, and in the
discharge of the mysterious processes of reproduction and of nutrition,
although the latter forms a convenient line of separation. Life in
plants goes on indefinitely; cuttings may be taken without injury to
their vigour and duration of life. The same may be said of some of the
lower forms of invertebrate life; for example, the hydra, the anemone,
and some other well-known animals, may be cut up, divided into several
parts, each one of which will form a new animal, provided a nucleus be
included in the section. Nevertheless, the organisation of the amœba
and the hydra is as complete for its purpose as that of man for his,
and the evidence of continuity forbids the drawing of hard and fast
lines, as was formerly done between the two kingdoms, the animal and
vegetable. The amount of similarity or agreement in the organisation
of animals is various. Animals indeed differ from each other in slight
points only, for the discovery of which the microscope must be brought
into requisition. Living matter in its earliest stage and simplest form
appears to the naked eye as a homogeneous structure, but when placed
under the highest powers of the microscope, it is seen not to be so.
But perhaps the most marked feature of the age has been the increasing
attention given to the study of the lower forms of life, using their
simpler structures and more diffuse phenomena to elucidate the more
general properties of living matter. To understand life we must
understand protoplasm. Of this there can be no doubt, as we have seen
in a previous chapter that a whole family, the Monera, consists of
this simple living, microscopic, jelly-like substance, which has not
even begun to be differentiated, as in the amœba, which has as yet no
special organs, and every speck becomes a mouth or a stomach, and which
can be turned inside out and shoot out tongues of jelly to move and
feel with. “Reproduction is the faculty most characteristic of life,
and sharply distinguishes the organic from the inorganic.” It is,
then, the corpuscles of protoplasm, called cells (cellulæ), which have
so much interest for the physiologist, and these, like the cytods, may
form independent organisms, which are then termed unicellular. Again,
cells form other cells, and a multicellular organism results, and goes
on increasing in geometrical progression. In the Vertebrata the cell
retains its characteristic spheroidal shape, as seen in Fig. 423, and
undergoes division by virtue of its living protoplasmic mass.
[Illustration: Fig. 423.
1. Newly formed cell structure; 2. Division of the nucleus; 3. It
changes its situation in the cell; 4. Subdivides and breaks up; 5.
Cell-walls increase in thickness; 6. Branch out into stellate cells; 7.
Two cells coalesce; 8 and 9. Become multicellular.]
_Epithelial Cells._--All free surfaces of the human body, both internal
and external, are to a very considerable extent covered by epithelium
cells. These cells are everywhere the same, but with modifications in
shape and arrangement. Epithelial cells are nucleated and always joined
by their surfaces or edges, without, on the external surfaces, the
intervention of connective tissue.
There are four essential varieties:--1. Tesselated; 2. Columnar; 3.
Spheroidal; 4. Ciliated; in all of which the nucleus remains remarkably
uniform in its characters, is either round or oval, and flattened
out, measuring 1/6000th to 1/4000th of an inch in diameter. They
are insoluble in acetic acid, colourless, or slightly tinted by the
structure with which they are in contact, and usually contain one or
more nucleoli with a few minute irregular granules, as represented in
Fig. 424.
The simplest and most commonly distributed variety is the tesselated,
known also as the scaly, squamous, pavement, and flattened
epithelium, always arranged in single layers, lining serous cavities,
many parts of the mucous membrane, and the interior of ducts and
blood vessels. Upon the external surface of the body it occurs in
superimposed layers, forming the “stratified epidermis.” To obtain
specimens of lamellar epithelium it is only necessary to collect a
little saliva, or pass a glass slide over the lining membrane of the
cheek, cover it with a thin cover glass, and examine it with a 1/4-inch
objective. Pavement epithelium is the elementary structure of hair,
nails, and horn.
[Illustration: PLATE XIX.
ANIMAL TISSUES.]
Columnar epithelium exists upon the mucous membrane of the stomach, on
the villi of the intestines, and in the several canals. It occupies
either a vertical or horizontal position, and may be detached in rows,
as shown in Plate XIX., No. 2, a section taken from the intestine of a
rabbit. This variety, when more highly magnified, as in Fig. 424, is
seen to consist of club-shaped nucleated cells, the thicker end being
turned towards the surface. The protoplasm of the cell is granular,
and the presence of minute vacuoles and fatty globules occupy a great
part of the space. The nucleus is now seen to contain a fine network.
At times the outer end of the cell is distended, as in Fig. 3. This
form of columnar epithelium (known as the “goblet” cell) presents a
close and remarkable resemblance to the cilio-flagellate “collared”
infusorial monad in its extended “wine-glass” form.
[Illustration: Fig. 424.
No. 1. Pavement epithelium, taken from an internal membrane; 2.
Columnar epithelium, from the intestine of a rabbit, showing central
fat globules, and at str a fine ciliated border; 3. A so-called
“goblet”-cell.]
Spheroidal epithelium is confined to the closed cavities of the body,
and in the internal structure of the ducts of secreting glands. The
cells are, for the most part, circular, although some are flattened
out at the sides in which they are in contact with each other (Plate
XIX., No. 1_a_). Specimens of this form may be taken from the internal
surface of one of the lower animals with a scalpel. The collected
matter must be placed in a drop of distilled water and examined with a
high power.
Ciliated epithelium is characterised by the presence of those fine
hair-like filaments (cilia) attached to the free surface of the
cell. During life, and for some time after death, the cilia are seen
to retain their constant waving motion. The cilia all move in one
direction and rhythmically, thus giving rise to the appearance of a
succession of undulations. Ciliated epithelium is found lining the
mucous membrane of the air passages and nasal ducts, and wherever it
is necessary to urge on a secretion by mechanical means, ciliated
epithelium exists. Specimens for examination are easily obtained
from the oyster, and with care will show the characteristic motion.
A portion of a gill separated from the mollusc will live on for a
considerable time if kept in a little of its natural secretion. The
parameciæ, rotifera, and all the ciliata, are furnished with cilia
as a means of locomotion and obtaining sustenance. By snipping off
a small piece from the gills of the mussel, always accessible to
the microscopist, and covering it over with thin glass to prevent
evaporation of the animal juices, its cilia will continue to work for
hours.
_Lymph and Blood_, Fig. 425 B, _a a_.--There are other cells in the
animal body which possess a certain amount of resemblance to those
confined to the more superficial structures--_i.e._, the lymph, chyle,
and blood. These fluids present in one respect a physical uniformity
of composition, and a resemblance in the size of their characteristic
corpuscles. Chyle contains besides the corpuscles of lymph, a quantity
of minute granules which imparts a white colour to the fluid.
Intermixed are oil globules, free nuclei, and sometimes a few red blood
discs. Chyle may be had for microscopic examination by squeezing a
little juice from the lymphatic gland of a sheep just slaughtered.
[Illustration: Fig. 425.--Human Blood Corpuscles and Crystals.
A. _a a._ Red blood corpuscles lying flat on the warm stage; _b b._
in profile; _c c._ arranged in rouleaux; _d_. crenated; _e_. rendered
spherical by water; I. leucocytes and white amœboid corpuscles; B.
Blood discs of fowl, red and white, others seen in convexity and with a
nucleus. Blood Crystals.--C. Hæmatin from human blood; D. Hæmatoidin;
E. Hæmin; F. Tetrahedral; G. Pentagonal; H. Octahedral crystals from
blood of mouse.]
_Blood Corpuscles_ or cells vary considerably in mammals, birds,
reptiles, and fishes. Fig. 102 (page 143) is a microphotograph of a
drop of blood magnified 3,500 times; and Fig. 425, A, shows both red
and white discs drawn to scale, magnified 1,200 diameters. The red
corpuscles of human blood are distinguished by their clearly defined
outlines and dark centres. Each disc is biconcave in form, and hence
the whole surface cannot be focussed at the same time. When the
circumference is well illuminated the centre is dark, but by bringing
the objective nearer to the object, the concavity of the disc is
brought into focus. It generally happens that blood corpuscles, on
being first drawn, run together, and present the appearance of rolls
of coins; or they may be scattered about over the field. There is a
considerable difference in the form of the discs; they are circular in
all mammals, except the camel, dromedary, and llama, these being oval.
In profile blood corpuscles are biconcave, their investing membrane
is homogeneous and elastic, and will readily move along the smallest
capillary vessels. There is no trace of a nucleus in the blood-discs of
the adult Mammalia, while in size they bear no proportion to the bulk
of the animal in whose blood-vessels they circulate. The corpuscles
of Mammalia in general are like those of man in form and size, being
either a little larger or smaller. The most marked exception is
the blood of the musk-deer, in which the corpuscles are of extreme
smallness, about the 1/12000th of an inch in diameter. In the elephant
they are large, about 1/2700th of an inch in diameter. The goat, among
common animals, has very small corpuscles, but they are, withal, twice
as large as those of the musk-deer. In the _Menobranchus lateralis_
they are of a much larger size than in any animal, being the 1/350th
of an inch; in the proteus, the 1/400th of an inch in the longest
diameter; in the salamander, or water-newt, 1/600th; in the frog,
1/900th; lizards, 1/1400th; in birds, 1/1700th; and in man, 1/3200th
of an inch. Of fishes, the cartilaginous have the largest corpuscles;
in gold-fish, they are about the 1/1700th of an inch in their longest
diameter.
The large size of the blood discs in reptiles, especially in the
Batrachia, has been of great service to physiologists by enabling them
to ascertain many particulars regarding structure which could not have
been otherwise determined with certainty. The value of the spectroscope
in the chemical examination of the blood has been already referred to.
See page 252.
White corpuscles or leucocytes (Fig. 425, I) differ materially from
the red. They are large, spheroidal, finely granular masses of about
1/2800th of an inch in diameter. In a cubic millimètre of human blood
there are about 10,000 white corpuscles. They have a lower specific
gravity than the red, have no cell-wall, and their substance mainly
consists of protoplasm. The internal granular appearance is now
believed to be due to a fine intercellular network having small dots
at the intersections of the web. In the meshes of the net a hyaline
substance is interspersed. They possess one or more nuclei; these are
seen on the application of a few drops of acetic acid. When examined
in a perfectly fresh state, especially if the glass slide be placed on
the warm stage of the microscope, they exhibit a spontaneous change
of shape, amœba-like, such movements being accordingly termed amœboid.
The movements referred to consist in the protrusion of processes
of protoplasm which are retracted and other processes protruded as
represented (Fig. 425, I). Both in human blood and in newts there are
colourless corpuscles which contain coarser granules than others; these
are called granular corpuscles. Some are shown near the amœboid bodies.
The white corpuscles are readily found in various tissues of the body,
as in the lymphatic glands. In inflammatory diseases these _leucocytes_
pass through the walls of the capillaries into the tissues, and form
morbid products, pus-cells.
Sections of blood discs are made by dipping a fine needle in a drop of
blood as it exudes from a prick of the finger and drawing thin lines
across the glass slip, allowing time to dry, and then cutting the lines
across in all directions with a razor. The loosened portions should be
removed with a camel’s-hair brush.
In birds, the blood discs are oval in shape and possess a nucleus,
shown in Fig. 425 B, in the blood of the fowl; this is rendered more
apparent on adding a drop of acetic acid. The blood of fishes is also
oval and nucleated, rather more pointed than that of birds. In reptiles
generally the red blood discs are large, oval, nucleated bodies, the
white corpuscles still preserving their invariable circular form and
granular appearance. In the salamander and proteus the discs attain to
their greatest size. In the former they measure 1/700th of an inch, and
in the latter 1/400th.
_Blood Crystals._--In addition to the elements described, the blood
contains various crystalline forms, represented in Fig. 425, C to H. In
connection with the micro-spectroscope (p. 253), the spectra of certain
blood crystals are given; although varying in different animals,
sufficient uniformity prevails as to render them characteristic. The
crystals are formed when a little blood is mixed with water on the
slide, allowing a short time for crystallisation. Near the edge of the
cover-glass, where crystals begin to form, they are more distinct, but
a high power is required for their examination. In human blood the
crystals are prismatic; in that of the guinea-pig, tetrahedral; in the
blood of the mouse, octahedral. Other forms may be obtained by the aid
of chemical reagents.
In human blood there are at least three distinct forms of crystals:
_Hæmatin_ is formed in normal blood, is made visible on the addition
of a little water to blood, or by agitation with ether, so as to
dissolve the cell-wall of the blood corpuscles, and allow the contents
to escape. A drop of blood will furnish crystals large enough to
be seen with a moderate power. _Hæmatoidin_ crystals are abnormal
products, found in connection with certain diseased conditions. These
crystals are seen as represented at D. _Hæmin_ crystals must be
regarded as artificial chemical products, the result of treating blood
with glacial acetic acid; the acicular crystals at E, reddish-brown in
colour, are artificially produced.
[Illustration: Fig. 426.
1. White fibrous or non-elastic tissue; 2. Yellow fibrous elastic
tissue.]
_Basement Membrane--Connective Tissue System._--Connective or areolar
tissue is present almost throughout the whole of the human body, and
serves to connect the various organs with one another, as well as
to bind together the several parts. The muscles are surrounded by a
connective tissue sheath; this penetrates into their substance, and
binds together fasciculi and fibres. The same tissue is present in the
skin and the mucous membranes; it also forms a sheath for the arteries,
veins, and nerves. It is plentifully supplied by blood-vessels, and
nerves pass through its substance. Microscopically, four different
elements can be clearly made out:--1. Connective tissue cells or
corpuscles; 2. White fibrous tissue; 3. Yellow fibrous tissue; 4.
Ground substance.
On examining the connective tissue cells of young animals, various
cells will be seen with fine granular contents, together with nuclei,
lying in spaces in the ground substance, some branched, others
flattened or rounded. Even tissues supposed to be homogeneous in
structure, are on staining seen to have connective tissue cells, such
as those represented in a section of the cornea of the eye (see p. 31).
In this case the connective tissue cells are termed corneal corpuscles;
the branched cells, it will be noticed, are united by branches.
The cells in the fibrous tissue of tendons are square or oblong, and
form continuous rows. White fibrous tissue is distributed throughout
the animal body, but in a variety of forms; it is found in the skin and
other membranes, and in all parts where strength and flexibility are
necessary. The structure of white and yellow fibrous tissues is shown
in Figs. 426 and 427.
[Illustration: Fig. 427.
1. White fibrous tissue lining the interior of the egg shell, with the
calcium carbonate removed by immersion in hydrochloric acid; 2. White
fibrous tissue, from the sclerotic coat of the eye.]
White fibrous tissue presents silver-lustre bundles, running for the
most part in parallel directions through and over the muscles and
tendons. For examination under the microscope, obtain a fragment of
fresh meat cut in the longitudinal direction; place it in water, and
tease it out with needles as directed in a former chapter. The smallest
fragment will suffice for examination under a quarter or one-sixth inch
objective. These filaments are exceedingly minute, measuring 1/3000th
to 1/2500th of an inch in diameter, and do not interlace through the
bundles, although they intersect each other occasionally. Transverse
sections may be made by drying a piece of tendon until it becomes
sufficiently firm to cut with a razor or microtome, and mounted as a
permanent specimen. From the cut ends of the fibres small dark points
will be seen, especially in the denser structure of the tendons; these
are termed “connective tissue corpuscles.”
Yellow elastic fibrous tissue is remarkable in contradistinction to
the white for its elasticity and capability of extension. It is found
on the coats of blood-vessels, between the vertebral arches, and in
quadrupeds it forms a strong elastic band, extending from the occiput,
throughout the spines of the vertebra, and enabling the animal to
support the head in the pendent position, without muscular exertion.
These fibres can only be separated from each other with difficulty, and
their elasticity is shown by a tendency to curl up. These yellow fibres
are somewhat coarser than the white, and they remain unaffected by
acetic acid of the ordinary strength. Elastic tissue is a constituent
of the skin, mucous, and serous membranes, and of the areolar or
cellular tissue.
In order to microscopically examine this structure, take a small
portion of the strong ligament of the neck of the ox, place it as
before in water, and tease it out with needles; place a fragment on
a glass slip, cover with a thin cover-glass, and submit it to a high
magnifying power. Transverse sections made as directed in the case of
white tissue will be seen to be hexagonal in form.
_Adipose Tissue._--Fat is found in many situations in the animal body,
and on examination is seen to consist entirely of vesicles, distributed
through a delicate membrane of connective tissue, shown in Plate XIX.,
Nos. 4 and 5. On pressure, the circular or oval form of the cells
becomes polyhedral; occasionally the fatty acids in the interior of the
vesicles crystallise, and give rise to a star-like appearance. For the
examination of adipose tissue, take a portion of the mesentery of any
small animal--a mouse, or rat.
_Retiform Tissue._--Adenoid, or retiform tissue, consists of a delicate
network of connective tissue corpuscles, joining their branches
together. This forms the stroma or framework of lymphoid tissue. It is
found in connection with all the lymphatic glands, spleen, &c. Plate
XIX., No. 3, _a b_, shows small sections of a lymphatic, together with
capillary vessels.
_Muscular Fibre._--There are two varieties of muscular fibre in the
body--_i.e._, striated, and non-striated. The striated is formed in
muscles attached to bony structures, as those of the arm and leg, and
in some of the soft structures, as the tongue, palate, œsophagus, in
short, all muscles under the control of the will. Striped muscle is of
a dull red colour and marked with peculiar longitudinal furrows on its
surface. Voluntary muscle consists of:--1, a connective tissue sheath;
2, fasciculi; 3, fibres and sarcolemma; 4, discs, fibrilla and sarcous
elements. These are shown in connection with other tissues in Plate
XIX., Nos. 11 and 12, and also in Fig. 428 (1, 2, 3).
[Illustration: Fig. 428.
1. Muscular fibre broken across, the fragments connected by the
connective tissue membrane × 100; 2. Fibre broken up into irregular
distinct bands: a few blood corpuscles distributed about × 200; 3. A
fasciculus of muscular fibre from leg of pig × 600.]
In Plate XIX., Fig. 11, the muscular fibre taken from the tongue of
a lamb shows the continuity of the upper portion with the connective
tissue membrane. In Fig. 12, a branching-out bundle of muscular fibre,
taken from the upper lip of the rat, is seen to end in stellate
connective cells. The delicate homogeneous sheath that binds the fibres
together is termed _sarcolemma_. This is readily seen in prepared
muscle of the frog and water-beetle, less plainly in man. Each muscle
is provided with a sheath of connective tissue; this surrounds it,
binds the fasciculi together, and supports the blood-vessels; it is
called the _perimysium_, and sends fine prolongations in between the
fibres, termed _endomysium_. The intervals seen on high amplification
between the dark striæ are called Kruse’s membrane. On breaking up the
striated structure it is resolvable into fibrillæ and furthermore into
discs.
[Illustration: Fig. 429.
1. Vertical section of epidermis; 2. Pigment cells from a lower layer
of cutis.]
Among mammalia the pig furnishes the best examples of muscle fibrillæ;
among insects the water-beetle and the thorax of the housefly. A
power of 600 or 800 diameters is required to separate the fibrillæ.
Blood-vessels are well supplied with striated muscle, but none of
their minuter branches penetrate the sarcolemma. The involuntary or
non-striated variety of muscular fibre exists in all parts of the body
where movements occur independently of the will, also in the ciliary
muscle and the iris of the eye, as well as in the middle coats of the
arteries. Non-striated fibres are pale in colour, prismatic in shape,
and easily flattened by pressure. In size, they vary from 1/7000th to
1/3500th of an inch in diameter, and are marked at short intervals by
oblong corpuscles.
_The Integument or Skin_ consists of epidermis or cuticle, dermis,
corium or cutis vera, sweat-glands, nails, hairs, sebaceous glands, and
numerous nerves and vessels. The epidermis forms a protective covering
over the whole surface of the body, and is moulded on to the surface
of the corium beneath, covering the ridges, depressions and papillæ.
It is made up of three principal layers: the horny layer or _stratium
corneum_, the most superficial, this consists of layers of flattened
cells, which are without a nucleus; the _stratum lucidum_, composed
of layers of nucleated cells, more or less indistinct in section; the
_rete mucosum_ or malpighian layer; is composed in its upper part of
layers of “prickle cells” and its inferior of a single stratum of
columnar cells. Pigment is principally found in the lowest layer, Fig.
429.
The gradations of colour in the skin are due to the granular contents
of the pigment cells. This is seen on steeping sections cut from the
skin of a negro in chlorine; the colour is discharged. In Plate XIX.,
No. 13, the pigment cells of the choroid coat of eye are shown. Here
the pigment is darker in colour, and its function is the absorption of
light and the prevention of disturbing effects occasioned by circles of
dispersion.
[Illustration: Fig. 430.--Vertical section of skin and subcutaneous
tissues, showing the sweat-glands and fat-globules, ducts passing
upwards to the epidermis or external cuticle. Magnified 250 diameters.]
_The Dermis_, or true skin, consists of an interlacing network of
connective tissue, yellow elastic tissue corpuscles, vessels, and
nerves. There are also small muscular fibres in connection with the
hair follicles, and beneath the subcutaneous tissues contain an
abundant supply of fat adipose tissue. Numerous ridges are seen on
the surface, especially on the palm of the hand and sole of the foot,
caused by rows of little elevations of the cutis vera, termed papillæ.
These are more or less conical, and contain a capillary loop, nerve,
and touch corpuscle, which serve to increase the sensitiveness of the
part, lodging a touch corpuscle in a favourable position for receiving
sensations of touch, Fig. 430.
Sweat glands are situated in the subcutaneous tissue, and consist
of fine tubes, which form the duct (seen in the section, Fig. 430);
these are continuous with a blind extremity, coiled up into a ball
one-sixtieth of an inch in diameter, and surrounded by a plexus of
capillaries to form the gland (Fig. 431, No. 2). Between the layer of
columnar cells and the limiting membrane is a layer of non-striated
muscle, and beneath the rite mucosum there are several layers of
polyhedral cells, and an external and internal limiting membrane; the
epithelium of the duct is at its mouth continuous with the epithelium
of the epidermis.
[Illustration: Fig. 431.
1. Blood vessels of papillæ supplied to cutis; 2. Perpendicular section
through the scalp, with two hair-sacs; _a._ epidermis; _b._ cutis; _c._
muscles of the hair follicles.]
_Nails_ consist of a root and body, the lunular of which is the
whitish portion of the body near the root, where the skin beneath is
less vascular than any other portion of the finger. The nail closely
resembles the epidermis, and consists of hard and thin layers of cells
on the surface, and round, moist cells beneath. Posteriorly the nail
fits into a groove which lodges its root. The part to which the nail is
attached is known as the nail-bed. The stratified appearance produced
by the coalescence of the cells, and their lying over each other, is
shown in Plate VII., No. 149, the toe of the mouse; while the special
arrangement of tissue is better seen under polarised light (Plate
VIII., No. 174).
_Hairs_ consist of a shaft and root. The shaft is cylindrical, and
covered with a layer of imbricated scales, arranged with their edges
upwards. The substance of the hair consists of fibres, or elongated
fusiform cells, in which nuclei are seen. There are present in some
hairs (Fig. 432) small air spaces or lacunæ. In the coarser hair of the
body there is a pith (medulla), occupied by small angular cells and fat
granules.
[Illustration: Fig. 432.
1. Single Hair-root and Shaft; 2. Vertical section, showing fibrous
character of the hair together with colouring matter, external edges
serrated; 3. Transverse section of human hair, medullary substance, and
central pith.]
The root of the hair is seen to dilate that it may fit more firmly into
the skin hair-follicle. The latter consists of two coats, an outer
and an inner, continuous with the epidermis, and this is called the
root sheath. The outer portion consists of three layers, formed of
connective tissue, blood-vessels, and nerves. The inner, or epidermic,
coat comes away when the hair is pulled out, and hence is called
the root sheath. This again is made up of two layers, the outer of
which corresponds with the horny layer, and is composed of flattened
cells. The bulbous root of the hair is connected with the papilla.
In the cat the tactile nasal hairs are very large. Small bundles of
involuntary muscular fibres connect the corium with the root, so that
in contracting they elevate or expand the hair.
[Illustration: Fig. 433.
1. Jointed hairs of Indian bat; 2. Hair of flying-fox, showing
imbricated scales; 3. Hair of mouse, showing pigment layers; 4. Hair of
a small beetle (Dermestes). × 250.]
The hair of the lower animals presents a diversity of structure,
especially on the outer surface, and with reference to the arrangement
of the scales. The hair of the Indian bat, for instance, consists of a
shaft invested with erectile scales, placed at regular intervals; these
stand out from the shaft, as in Fig. 433, No. 1. This form of scale
varies considerably in the different species of these animals, and a
portion of hair near the root is nearly divested of scales. Many of
the scales are not unlike those of certain of the insect tribe, seen
in that of Dermestes, No. 3, while the hair of the mouse has a series
of transverse imbricated scales arranged as tiles on a house, due to
accumulated pigment. Hairs taken from various animals form interesting
objects of study for the microscope, as already noticed. Other hairs
are shown in Fig. 434. No. 1 is a transverse section of a hair from
the ant-eater; the central part consists of air-cells, the outer of a
granular pith. No. 2 is a transverse section of hair of peccary, with
a diversified arrangement of the cortical envelope, sending outward
a set of radial prolongations and air-cells; this kind of structure
is also found in the quills of the porcupine. No. 3 is a transverse
section of a hair of the elephant, which shows a combination of a
number of tubes united together, somewhat resembling the arrangement
of the hoof-horn of some of the ruminants, and the denser horny growth
on the snout of the rhinoceros, No. 4. The curious modification of
these horny structures is seen in the horns of other animals, and which
may be likened to a bundle of hairs. On making a transverse section,
as in Fig. 434, and submitting it to polarised light, on rotating the
analyser, the dark central spot shown is replaced by a bright one with
a play of colours due to the interference of light (Plate VIII., No.
178). The scales of fish are also of interest (Fig. 435). These have
been shown to afford an unerring guide in the classification of fishes
and in the examination of their fossil remains. As a class of objects
for the microscope, they are found to be both curious and beautiful.
Plate VIII., No. 176, is a scale of the grayling, seen under polarised
light.
[Illustration: Fig. 434.
1. Hair of ant-eater; 2. Hair of peccary; 3. Hair of elephant; 4. Horn
of Rhinoceros.]
[Illustration: Fig. 435.--Fish Scale (Sole).]
Of the harder outgrowths of the dermal structures, the teeth afford the
chief example among animals. The rough anatomy of the tooth in mankind
consists of a crown, that projects from the gum; a root, or fangs,
fixed in a socket of the jawbone, and a short intermediary neck. Each
tooth is supplied with an artery and nerve, and has a central cavity
filled with a soft, vascular, sensitive substance, the pulp. On making
a vertical section of a tooth, we recognise the several structures in
the order of, pulp, crusta, petrosa, dentine, and enamel. A section
through a human molar tooth (shown in Fig. 436) will convey some idea
of the arrangement of the denser structures referred to above.
[Illustration: Fig. 436.--Sections of Human Molar Tooth (magnified 50
diameters). 1. Vertical section; 2. Horizontal section.]
Blandin was the first to demonstrate that teeth are developed in the
mucous membrane, similar to that of hair and nails. Teeth are formed in
grooves of the mucous membrane, and subsequently converted into closed
sacs by a process of involution, and their final adhesion to the jaw
is a later process. It is very generally conceded that teeth belong
to the _muco-dermoid_, and not to the periosteal, series of tissues;
that, instead of standing in close relation to the endo-skeleton, they
are part of the dermal or exo-skeleton; their true analogues being the
hair, and some other epidermic appendages. Huxley proved that, although
teeth are developed in two ways, they are mere varieties of the usual
mode in the animal kingdom. In the first, which is typified by the
mackerel and the frog, the pulp is never free, but from the first is
inclosed within the capsule, seeming to sink down as fast as it grows.
In the other, the pulp projects freely at one period above the surface
of the mucous membrane, becoming subsequently included within a capsule
formed by the involution of the latter; this occurs in the human
subject. The skate offers a sort of intermediate structure.
[Illustration: Fig. 437.
1. Section of a cusp of the posterior molar of a child. The inner
outline represents it before the addition of acetic acid--the outer
afterwards, when Nasmyth’s membrane _g_ is seen raised up in folds;
_f._ the enamel organ; _c._ the dentine; the central portion being
filled with pulp. 2. Edge of the pulp of a molar cusp, showing the
first rudiment of the dentine, commencing in a perfectly transparent
layer between the nuclei of the pulp and the _membrana preformativa_.
3. Nasmyth’s membrane detached from the subjacent enamel by acetic
acid. 4. Stellate-cells of the enamel organ. 5. Tooth of frog, acted
on by dilute hydrochloric acid, so as to dissolve out the enamel and
free Nasmyth’s membrane. The structure of the dentine _e_ is rendered
indistinct. At the base, Nasmyth’s membrane is continued over the bony
substance at _z_, in which the nuclei of the lacunæ are visible. (After
Huxley.) 6. Decalcified tooth-structure; _a._ the dentine; _b._ enamel
organ; _c._ enamel; _d._ Nasmyth’s membrane.]
The _enamel_ forms a continuous layer, and invests the crown of the
tooth; it is thickest upon the masticating surface, and decreases
towards the neck, where it usually terminates. The external surface
of the enamel appears smooth, but is always marked by delicate
elevations and transverse ridges, and covered by a fine membrane
(Nasmyth’s membrane), containing calcareous matter. This membrane is
separable after being subjected to hydrochloric acid; it then appears
like a network of areolar tissue, shown in Fig. 438, No. 6; Huxley’s
“_calcified membrana_,” which commence at the pulp cavity, and pass up
to the enamel.
[Illustration: Fig. 438.--Tooth Structure.
1. Longitudinal section of superior canine tooth, exhibiting general
arrangement, and contour markings, slightly magnified; 2 and 3.
Portions from same, highly magnified, showing the relative position of
bone-cells, cementum at 2, dentine fibres, and commencement of enamel
at 3; 4. Dentine fibres decalcified; 5. Nasmyth’s membrane separated
and the calcareous matter dissolved out with dilute acid; 6. Cells of
the pulp lying between it and the ivory; 7. A transverse section of
enamel, showing the sheaths of fibres, contents removed, and magnified
300 diameters.]
Czermak discovered that the curious appearances of globular
conglomerate formations in the substance of dentine depend on its
mode of _calcification_ and the presence of earthy material; and he
attributed the contour lines to the same cause. Contour markings vary
in intensity and number; they are most abundant in the root, and most
marked in the crown. Vertical sections exhibit them the best; as Fig.
440, No. 1. In preparing a specimen, first make the section accurately,
then decalcify it by submersion in dilute hydrochloric acid; dry it and
mount in Canada balsam; place the specimen in the hot chamber for some
time to soak in the fluid resin before it cools. The white opacity at
the extremity of the contour markings gives the appearance of rings to
the tooth-fang.
“The tooth-substance appears,” says Czermak, “on its inner surface, not
as a symmetrical whole, but consisting of balls of various diameter,
which are fused together into a mass with one another in different
degrees, and in which the dentine tubes in contact with the germ
cavity terminate. By reflected light, _dark-ground_ illumination, one
perceives this stalactite-like condition of the inner surface of the
tooth-substance very distinctly, by means of the varied illumination of
the globular elevations, and by the shadows which they cast.” To see
this structure to advantage the preparation should be made from a tooth
root, the growth of which is not complete. With such preparations,
the ground-substance of the last formed layer of the tooth-substance
is seen to be, at least partly, in the form of globular masses, fused
together with those of the penultimate layers.
The cementum is the cortical layer of osseous tissue, forming an outer
coating to the fangs, which it sometimes cements together. Its internal
surface is intimately united with the _dentine_, and in many teeth it
would appear as if the earliest determined arrangement of the fibres
of the dentine started from the _canaliculi_, as they radiate from the
lacunæ in the cement. The inter-lacunar layer is often striated, and
exhibits a laminated structure: sometimes it appears as if Haversian
canals were running in a perpendicular direction to the pulp cavity.
The _canaliculi_ frequently run out into numerous branches, connecting
one with another, and anastomising with the ends of the dentine fibres.
The thick layers of cement which occur in old teeth show immense
quantities of aggregated lacunæ of an irregular and elongated form.
[Illustration:
Fig. 439.--Transverse section of Tooth of Pristis, showing orifices of
medullary canals, with systems of radiating fibres (_tubuli_) analogous
to the Haversian canals in true bone.]
_Compact Tissues, Cartilage and Bone._--Cartilage is a bluish or
yellowish-white, semi-transparent, elastic substance, without vessels
or nerves, and surrounded by a membrane, termed pericondrium, of a
dense fibrous nature. That kind, however, known as articular cartilage,
receives a layer of epithelium from the synovial membrane, but this
is confined to marginal portions, in consequence of the central wear
which occurs as soon as the parts are subjected to friction, during
the movement of the limbs. Cartilage covers the ends of all bones in
apposition to form joints, and thus lessens the effects of concussion.
Besides the ordinary kind of cartilage, temporary and permanent, there
are two modifications of the tissue, confined to certain portions of
the body: cellular cartilage, composed of cells lying close together,
in a mesh formed of fine fibres; and fibro-cartilage, cells distributed
in a matrix of fibrous tissue.
Examined with a low power, cartilage appears to be homogeneous in
structure, studded over with numerous round, oval, oblong, semilunar,
and irregular-shaped corpuscles, as seen in Plate XIX., No. 8, a
vertical section of animal cartilage, arranged in columns, and
condensed at the lower surface previous to its conversion into bone.
The greater opacity of this portion is owing to the increase of osseous
fibres, and the multiplication of oil globules, and the intercellular
spaces becoming filled with vessels. No. 9 shows a small transverse
section of the same, with a further change of the cartilage cells at
_a_ into bone cells, and at _b_ with the characteristic canaliculi and
lacunæ. No. 7 further shows a section of the large tendon fixed to the
back of the heel of the foot, near the juncture of the tendo-Archillis
with the cartilage. For the examination of these several changes a high
power is necessary, and for the purpose pieces taken from the ox may be
easily obtained from the butcher, and fine sections cut with a razor
parallel to the surface.
[Illustration: Fig. 440.
1. Cartilage from a mouse’s ear closely resembling vegetable tissue
×200; 2. Cartilage from rabbit’s ear, with nucleated cells embedded in
matrix; 3. Cartilage from the end of a human rib ×300.]
The better specimens for microscopical examination are those taken from
very young animals, in whom the ossific process is still incomplete. In
order to examine cellular cartilage, the ear of the mouse should be
taken and just dried sufficiently to enable fine sections to be cut by
the microtome transversely (Fig. 440).
Cartilage forms the entire skeleton of a certain number of fishes, as
the skate, lamprey, ray, shark, &c., the cells of which are embedded
in a matrix of granular matter, which has been properly termed
intercellular. The nearest approach to ossification of cartilage in
fishes is that of the cuttle-fish; in this stellate cells are freely
distributed, as shown in Fig. 441, No. 3.
[Illustration: Fig. 441.
1. Cartilage from the head of the skate, cells filled with nuclei;
2. Cartilage from frog, oblong cells with nuclei; 3. Cartilage from
cuttle-fish, with stellate cells, × 200.]
White fibro-cartilage occurs between the bodies of the vertebræ as a
connecting medium. In this kind the cells are more widely distributed,
specimens of which may be taken from the central portion of an
interarticular disc of any animal. The oval or circular corpuscles will
be seen surrounded by an abundance of fibrous tissue.
An acquaintance with the degeneration of the textures with which we
have been dealing may be of service to the student, as he may, in the
course of his examination, meet with an abnormal condition altogether
different to those described. The process of degeneration is usually
a slow one, except in the case of fatty infiltration, an example of
which is furnished by the fatty degeneration of the liver in Strasburg
geese. Muscular tissue is very prone to fatty degeneration, and fatty
heart is often met with. Calcareous degeneration of the muscles,
ligaments, and cartilages, as well as morbid deposits, are not at all
uncommon in these structures. In Plate XIX., No. 9, a small section
is given of an enchondroma, and in which the round or ovoid cells of
the cartilage are seen degenerated and converted into granular masses
of a calcareous nature. Fig. 442 is a somewhat more highly magnified
section of a calcareous or morbid growth, taken from a human subject in
which a morbid growth was seen to be gradually destroying the bone and
cartilage cells.
[Illustration:
Fig. 442.--Cartilage taken from a diseased finger, in which both
cartilage and bone were in a state of degeneration.]
_Bone._--Bone is a hard unyielding structure, and which in the
vertebrata forms the skeleton of the adult. It is the framework for the
support of the soft tissues of the body, and forms various cavities for
the reception of important organs, as the brain, spinal cord, eyes,
heart and lungs, and acts as levers for the action of the muscles and
joints. The partial elasticity of bone is seen in the ribs, and the
rebound when the skull is dropped on the ground. Bone consists of
earthy and animal matters intimately combined; the removal of either,
however, does not destroy the form of the bone, if the process of
separation be carefully conducted. The earthy constituents may all
be dissolved out by hydrochloric acid, but the form of the bone is
preserved in its minute particular, and in this state sections may
be cut for microscopical examination. If allowed to become dry it
shrivels, and assumes the density of horn. The interior of a bone is
of a spongy or cancellated structure, particularly at the ends. The
outer portion of the bone is more dense than the internal part. The
study of bone should commence with sections of the softened structure.
Directions for making sections of bone are given in the chapter on
Practical Microscopy.
[Illustration: PLATE XX
VERTEBRATA, BONE STRUCTURE.]
The intimate structure of bone will be studied in connection with Plate
XX. Two series of lamellæ may be demonstrated in bone after maceration
in acid, a larger system surrounding the medullary canal, and a smaller
surrounding the Haversian canals, both of which are seen in Nos. 1 and
2. In macerating bones, the lamellæ of the layer concentric system
may be peeled off in layers; these are seen to be pierced with fine
apertures, caused by the canaliculi. In some parts larger apertures
are seen through which bundles of fibres pass, pinning, as it were,
the several layers together; these are the perforating fibres. The
outermost of the layers, being near the periosteum, the membrane
covering the bone, are termed periosteal layers; the innermost, being
close to the canal, are called medullary layers. No. 1 is a transverse
section of a flat bone, the clavicle, and it shows the Haversian
canals, varying in size from 1/2000th to 1/200th of an inch in
diameter, the largest being near the medullary canal. In shape they are
round, oval, or oblong, according to the line of section. Each canal
is surrounded by rings, none of which are complete, and running one
into the other at various parts. Under a higher power, those irregular
shaped bodies termed lacunæ, with fine radiating fibres, are seen to be
smaller canals, canaliculi.
By means of this complete and intricate distribution of the canals of
the Haversian system, the nutritive fluids pass into the most compact
parts of the osseous tissue. Longitudinal sections of the long bones
show these canals as continuous branching-out cells.
In many of the lower animals the bony structure differs from those of
man, as will be seen in Plate XX. No. 3 shows a transverse section of
the femur, or leg-bone of an ostrich, magnified ninety-five times,
in which the Haversian canals are much smaller and more numerous,
and many of them run in the transverse direction. No. 4, again, is
a transverse section of the humerus, or fore-arm bone of a turtle
(_Chelonia mydas_). This exhibits traces of Haversian canals, with a
slight tendency to a concentric arrangement of bone-cells around them,
the bone-cells being large and numerous, and occur, for the most part,
in parallel rows. In No. 5, a horizontal section of the lower jaw-bone
of a conger-eel exhibits a single plane of bone-cells arranged in
parallel lines. There are no Haversian canals present, and when this
specimen is contrasted with that of No. 4, it will be noticed that
the canaliculi given off from each of the bone-cells of this fish are
very few in number in comparison with that of the reptile. No. 6 is a
section of a portion of the cranium of a siren (_Siren lacertina_),
remarkable for the large size of the bone-cells, and of the canaliculi,
which are larger in this animal than in any other yet examined; and as
in the preceding specimen, no Haversian canals are present. No. 7 is a
section of bone taken from the exterior of the shaft of the humerus of
a Pterodactyle; this exhibits the elongated bone-cells characteristic
of the order Reptilia. No. 8 is a horizontal section of a scale, or
flattened spine, from the skin of a Trygon, or sting ray; this exhibits
large Haversian canals, with numerous wavy parallel tubes, like those
of dentine, communicating with them. This specimen shows, besides wavy
tubes, numerous bone-cells, whose canaliculi communicate with the
tubes, as in dentine.
The following points may be noted with regard to the several sections
of bone described. That of the bird, for instance, contrasted with that
of the mammal, exhibits the following peculiarities: the Haversian
canals are more abundant, much smaller, and often run in a direction
at right angles to that of the shaft, by which means the concentric
laminated arrangement is in some cases lost; the direction of the
canals follows the curve of the bone; the bone-cells are much smaller
and more numerous; while the number of canaliculi sent off from the
cells is less than in those of mammals. No. 3 is the average length of
a bone-cell of the ostrich, 1/2000th of an inch, in breadth 1/6000th.
In the Reptilia, the bones may be either hollow, cancellated, or solid;
and their specific gravity is less than that of birds or mammals. The
short bones of most of the chelonian reptiles are solid, and the long
bones are either hollow or cancellated; the ribs of the serpent-tribe
are hollow, the medullary cavity performing the office of a Haversian
canal; the bone-cells are accordingly arranged in concentric circles
around their canals. The vertebræ of these animals are solid; and
the bone, like that of certain birds, is remarkable for density and
whiteness. When a transverse section is taken from one of the long
bones, and contrasted with that of a mammal or bird, the difference
will be noticed; there are very few, if any, Haversian canals, and
these are large; and at one view, in the section, No. 7, the canals
and bone-cells are arranged both vertically and longitudinally. The
bone-cells are remarkable for the great size to which they attain; in
the turtle they are 1/375th of an inch in length, the canaliculi are
extremely numerous, and are of a size proportionate to that of the
bone-cell.
In fishes a greater variation occurs in the minute structure of
the skeleton than in either of the three preceding classes. A rare
structure is that of the sword of the sword-fish (Istiophorus). In
this, Haversian canals and a concentric laminated arrangement of the
bone are found, but no bone-cells. The Haversian canals, when they are
present, are of large size, and very numerous, and then the bone-cells
are, generally speaking, either absent or but few in number, their
place being occupied by tubes or canaliculi, which are often of a
very large size. The bone-cells are remarkable for their graduate
figure, and the canaliculi derived from them are comparatively few in
number. In a thin section of the scale of an osseous fish, the cells
lie nearly all in one plane, and the anastomoses of the canaliculi are
more distinctly seen; in the hard scales of many, as the Lepidosteus
and Calicthys, and in spines of the Siluridæ, the bone-cells are well
differentiated. In the true bony scales comprising the exo-skeleton of
cartilaginous fishes the bone-cells are seen in great numbers.
Now, if we proceed at once to the application of the facts which have
been laid down, and make a fragment of bone of an extinct animal
the subject of investigation, it will be found that the bone-cells
in Mammalia are tolerably uniform in size; and if we take 1/2000th
of an inch as a standard, the bone-cells of birds fall below that
standard; but the bone-cells of reptiles are much above either of
the two preceding, while those of fishes are essentially different,
both in size and shape, and are not likely to be mistaken for one or
the other; so that the determination of a minute yet characteristic
fragment of fishes’ bone is a task easily performed. If the portion of
bone does not exhibit bone-cells, but presents either one or other of
the characters indicated, the task of discrimination is equally easy.
We have now the mammal, the bird, and the reptile to deal with. In
consequence of the very great size of the cells and their canaliculi
in the reptile, a portion of bone of one of these animals can readily
be distinguished from that of a bird, or a mammal. The only difficulty
lies between these two last; but, notwithstanding that on a cursory
glance the bone of a bird appears very like that of a mammal, there are
certain points in their minute structure in which they differ; and one
is the difference in size of the bone-cells. To determine accurately,
therefore, between the two, we must, if the section be a transverse
one, also note the comparative sizes of the Haversian canals, and
the tortuosity of their course; for the diameter of the canal bears
a certain proportion to the size of the bone-cells, and after close
examination the eye will readily detect differences.
[Illustration: Fig. 443.
1. A portion of the web of frog’s foot, spread out and slightly
magnified to show distribution of blood-vessels; 2. Is a portion
magnified 250 diameters to show the ovoid form of the blood discs in a
vessel, beneath which hexagonal nucleated epithelium cells appear.]
_Arteries and Veins._--The circulation of the animal frame is
maintained by arteries, veins, and capillaries. The arteries are
elastic and contractile tubes; these convey the blood from the heart
to the capillaries. The larger arteries are exceedingly elastic,
but feebly contractile on account of the muscular tissue in their
walls. The veins ramify throughout the body, are more numerous than
the arteries, and of greater capacity. They usually accompany the
arteries and correspond to them in structure, the larger veins
possessing semi-lunar valves; these project into their interiors, and
thus prevent the regurgitation of the blood. They have four coats,
consisting of areolar tissue, yellow fibres combined with muscular
fibres, and white fibrous tissue, two layers of yellow fibres arranged
longitudinally, and a single layer of epithelial cells. Intermediate
between the arteries and veins there are exceedingly fine tubes, termed
capillaries, in which the arteries terminate, and from which the veins
arise. These are composed of a fine homogeneous membrane, with here and
there a nucleus. The capillary circulation of the blood is readily seen
in the tail of the newt and the foot of the frog, Fig. 443.
[Illustration: Fig. 444.--A network of capillaries.]
A network of capillaries conveying blood to the lungs, and ramifying
throughout the structure, is shown in Fig. 444, and in Plate XIX., No.
6, the termination of a capillary of a blood-vessel in the fat-cells of
the human body. Plate VII. illustrates the distribution of the arteries
and veins to various parts of the animal body. This coloured plate,
however, is designed to show the value of injected preparations in
the delineation of animal structures. By thus artificially restoring
the blood and distending the tissues, a much better idea is obtained
of the relative condition of parts, the appearance presented by the
erectile papillæ, &c. In the section of foot of mouse (No. 149), the
bone is seen surrounded by its vascular supply, arterial and venous;
in No. 150, the papillæ of the tongue are distended and seen erect; in
No. 152, a vertical section of the fungi-form papillæ on the tongue of
cat, with capillary loops passing into them, is demonstrated; in No.
151, the vertical section of brain of a rat, the vascular supply is
shown; No. 153, the malpighian tufts (circular bodies) and arteries
ramifying about the structure; in No. 154, the vertical section through
the intestine of the rat, shows villi (arteries and veins) surmounted
by epithelium, and supported on a layer of the mucous membrane; in
No. 155, the vascular supply sent to the roots of the whisker of the
nose of the mouse; in No. 157, a tangential section cut through the
several textures, the sclerotic coat and retina of the eye of a cat
is clearly made out although not highly magnified; again, in No. 156,
the beautiful vascular arrangement of the internal gill of the tadpole
could scarcely be so strikingly illustrated in any other way; while in
the central, No. 158, the vascular system throughout the whole of the
body of a fully developed tadpole, with the way in which the blood is
carried from the remotest part of the tail to the heart, and sent to
the gills, the brain, &c., it is quite unnecessary to enlarge upon.
These are seen under a low power, but for the purpose of studying
the basement membrane, together with the intimate association and
termination of the nerves accompanying the arteries and veins, it is
absolutely necessary to resort to a staining process, and cutting fine
sections with the microtome. Small portions of a nerve may be cut off
with fine scissors, teased out with needles, and a drop of acetic
acid added to render the sheath more transparent; in a few seconds
the connective tissue corpuscles will be brought into view. For the
microscopical examination of nerve-fibrillæ take a small section from
the leg of a frog, and tease it out in blood serum or white of egg. In
size the fibrillæ vary, even in the same nerve, from the 1/12000th to
the 1/1500th of an inch in diameter.
[Illustration: Fig. 445.]
To show the circulation of the blood in the frog’s foot, and without
causing the animal pain or much inconvenience, it is better to
enclose it in a black silk bag, and draw out the foot as shown at _a
a a_, Fig. 445. The bag provided should be from three to four inches
in length, and two and a half inches broad, shown at _b b_, having
a piece of tape, _c c_, sewn to each side, about midway between the
mouth and the bottom, and the mouth itself capable of being closed
by a drawing-in string, _d d_. Into this bag the frog is placed, and
only the leg which is about to be examined kept outside; the string
_d d_ must then be drawn sufficiently tight around the small part of
the leg to prevent the foot from being pulled into the bag, but not to
stop the circulation; three short pieces of thread, _f f f_, are now
passed around the three principal toes; and the bag with the frog must
be fastened to the plate _a a_ by means of the tapes _c c_. When this
is accomplished, the threads _f f f_ are passed either through some of
the holes in the edge of the plate, three of which are shown at _g g
g_, in order to keep the web open; or, what answers better, in a series
of pegs of the shape represented by _h_, each having a slit, _i_,
extending more than halfway down it; the threads are wound round these
two or three times, and then the end is secured by putting it into the
slit _i_. The plate is now ready to be adapted to the stage of the
microscope: the square opening over which the foot is secured must be
brought over the aperture in the stage through which the light passes
from the mirror.
The tadpole circulation is readily seen by placing the creature on its
back, when we immediately observe the beating heart, a bulbous-looking
cavity, formed of delicate, transparent tissue, through which the blood
alternately enters by one orifice and leaves by a more distant exit.
The heart, it will be noticed, is enclosed within its pericardium,
this being the more delicate part of the creature’s organisation.
The binocular microscope should be used for viewing the circulation.
Passing along the course of the great blood-vessels to the right
and left of the heart, the eye is arrested by a large oval body,
of a more complicated structure. This is the inner gill, formed of
delicate, transparent tissue, traversed by arteries, and a network
of blood-vessels. It is almost unnecessary to say the tadpole has a
respiratory and circulatory system resembling those of fishes.
In nearly all fish the heart has but two cavities, an auricle and
ventricle; the blood is returned by the veins to the auricle, passes
into the ventricle, and is then transmitted to the gills, where, being
exposed to the air contained in the water, it becomes deprived of
carbonic acid, aerated, and rendered fit to breathe. In the reptile we
find a modification of plan. The heart has three cavities, two auricles
and one ventricle; by this contrivance there is a perpetual mixture in
the heart of the impure carbonized blood which has already circulated
through the body, and flows into the ventricle from the _right_
auricle, with the purer aerated blood returned from the lungs, which
flows at the same instant into the ventricle from the _left_ auricle.
For the purpose of subsequent observations the tadpole should be
selected at a period in which the skin is perfectly transparent,
otherwise the appearances already described of the form and situation
of the heart, and the three great arterial trunks (proceeding right and
left), will not be clearly made out. The anatomical arrangement of the
vessels will be seen to be closely connected with the corresponding
gill, the upper one (the _cephalic_) running along the upper edge of
the gill, giving off, in its course, a branch which ascends to the
mouth, with its accompanying vein; this is termed the _labial_ artery
and vein. The cephalic artery continues its course around the gill,
until it suddenly curves upwards and backwards, and reaches the upper
surface of the head, when it dips down between the eye and the brain.
It must not be supposed that this can be made out in the average
tadpole, the obstacle to which is the large coil of intestines, usually
distended with dark-coloured food. This must first be reduced by making
your tadpole live on plain water for some days. Plate VII., No. 158,
affords a view of the vessels obtained under the influence of low diet,
and whereby we are enabled to trace the course of the three large
arteries. The third trunk, traversing the lung, is seen to emerge from
the lower edge and descend into the abdomen to form the great abdominal
aorta. A small half-starved tadpole shows the heart beating and the
blood circulating, but the latter is quite colourless, not a single red
globule visible anywhere. The heart is a colourless globe, the gills
two transparent ovals, and the intestines a colourless, transparent
coil. Through the empty coil the artery is seen on either side leaving
the gills, and converging towards the spine, and uniting to form the
abdominal aorta, the large central vessel coloured red in the figure.
After the aorta has supplied the abdominal viscera, a prolongation,
or _caudal_ artery is seen descending to the tail, the all-important
organ of locomotion in the tadpole. This artery, entering the root
of the tail, is imbedded deeply in the flesh, whence it emerges, and
then continues its course, closely accompanied by the vein, to within
a short distance of the extremity, where, being reduced to a state
of extreme fineness, it terminates in a capillary loop, composed of
the end of the artery and the beginning of the vein. The artery, in
its course, gives off branches continually to supply the neighbouring
tissue. The blood-current in the tail is often seen, even in the
main artery or vein, to be sluggish. This occurs independently of
the heart, which will continue to beat as usual; it happens, because
the circulation in the tail depends very much on the motion of the
organ. When this is suspended (as in the confined tadpole under the
microscope), the blood moves sluggishly, or stops, till the tail
regains its freedom and motion, when the activity of the current is
restored.
Having traced the arterial system which conveys the blood from the
heart to the extremities, we will now note its return by the veins back
again to the heart.
The caudal vein runs near the artery during the greater part of its
course, with its stream of blood _towards_ the heart. This stream is
swollen by perpetual tributaries from numerous vessels. As the vein
approaches the root of the tail it is inclined towards the artery,
and diverges from it at the point of entering the abdomen. Here it
approaches the kidneys and sends off branches, while the main trunk
continues its course onward; and, passing upwards behind a coil of
intestine, it approaches the liver, and runs in a curved course along
the margin of that organ. The blood is now seen to enter the vena
cava by several channels, that converge towards the great vein as it
passes in close proximity to the organ. Beyond the liver the vena cava
continues its course upwards and inwards to its termination in the
sinus venosus or rudimentary auricle of the heart. This termination is
the junction of not less than six distinct venous trunks, incessantly
pouring their blood into the heart. The circulation in the fringed lips
forms a most complicated network of vessels, out of which proceeds
a vein corresponding to the artery already traced. This descends in
a direct course till it joins the principal vein of the head, which
corresponds to the _jugular_ in the mammalia.
Thus it will be seen the blood is driven by the heart into each inner
gill through three large blood-vessels, which arise directly from the
_truncus arteriosus_, and may be called the _afferent vessels of the
gill_. In Plate VII., No. 156, an enlarged view of a gill is shown.
On closer examination “each _internal gill_ or entire branchial organ
is seen to consist of cartilaginous arches, with a piece of additional
framework of a triangular form, stretching beyond the arches, composed
of semi-transparent, gelatinous-looking material. These form the
framework of the organ and support upon their upper surface the three
rows of crests with their vascular network, and the main arterial and
venous trunks lying parallel to and between them. The three systemic
arteries arising, right and left, from the _truncus arteriosus_,
enter each gill on its cardiac side, and then follow the course of
the crests, lying in close proximity to them. The upper of these
branchial arteries runs alone on the outside of the upper crest, and
another branch leaving the trunk and passing into the network of the
crest, whence a returning vessel may be traced carrying back the blood
_across_ the branchial artery, and to a vessel lying close to and
taking the same course as the artery itself. Carrying the eye along
the latter vessel we find, at a short distance from the first of these
crest branches, a second, leaving the main trunk and entering the
crest, when a corresponding returning vessel conveys the blood across
the arterial trunk into the vessel lying beside it, as in the former
instance. A number of these branches may be traced from one crest to
the other. But it is now seen that the trunk from which these arterial
branches spring diminishes in size as it proceeds in its course (like
the gill artery in fishes), while the vessel running parallel to it
and receiving the stream as it returns from the crest enlarges to some
extent. Thus, the artery or _afferent_ vessel which brings the blood
to the gill is large at its entrance, but gradually diminishes and
dwindles to a point at the opposite end of the crest; while the venous
or _efferent_ vessel, beginning as a mere radical, gradually enlarges,
and thus becomes the trunk that conveys the blood out of the gill to
its ultimate destination. This vessel is the _upper branchial vein_ so
long as it remains in contact with the gill; subsequently it changes
its name on leaving the gill and as it passes upwards for distribution
to the head, when it is designated the _cephalic artery_. The _middle
branchial artery and vein_ proceed in like manner in connection with
the middle crest, and the _lower artery and vein_ in connection with
the lower crest. The middle and lower venous trunks, having reached the
extremity of the crests, curve downwards and inwards, and leave the
gill. The former trunk, converging towards the spine, meets its fellow,
and with it forms the _ventral aorta_. The latter gives origin to the
_pulmonary artery_, and supplies also the integuments of the neck.
Curious and interesting is the final stage of the metamorphosis, when
the waning tadpole and incipient frog coexist, and are actually seen
together in the same subject. The dwindling gills and the shrinking
tail--the last remnants of the tadpole form--are yet seen, in company
with the coloured, spotted skin, the newly formed and slender legs, the
flat head, the wide and toothless mouth, and the crouching attitude of
the all but perfect reptile.”[87]
To observe the circulation and how it is carried on during life in the
gills, the outer covering must be carefully raised, or even stripped
off. This will be better accomplished by putting the tadpole under the
influence of cocaine or chloroform--a drop of the fluid is sufficient
for the purpose.
The metamorphosis in the embryo of the frog is by no means exceptional.
The ascidian begins life in the form of a tadpole, with a muscular
tail; subsequently it fixes itself by its head to a rock, and its tail
disappears. The changes the tadpole of the frog passes through are in
every respect, except in one or two minor details, similar to those of
adult amphibia which pass their whole lives in water. The newly-hatched
flat-fish is symmetrical, an eye being placed on each side of its head,
with the adults of other fishes. The fœtal whale has well-developed
hind limbs, and which, after passing into a condition almost perfect in
proportion to the rest of the body, gradually dwindle away again to the
merest rudimentary structures. In all these, and a number of similar
cases, it is seen that the earlier condition of existing animals
represents, and is in agreement with that of its adult ancestor of a
remote period in the past. Collected facts bearing upon this question
have been made the groundwork of a theory of hereditary properties in
the germ, and a disposition to go through the same phases of life as
the parent.