the pineal gland, unless the view of Götte be regarded as such. It
appears to possess in all forms an epithelial structure, but, except
at the base of the stalk (infra-pineal process) in Mammalia, in the
wall of which there are nerve-fibres, no nervous structures are
present in it in the adult state.
The pituitary body. Although the pituitary body is not properly a
nervous structure, yet from its intimate connection with the brain it
will be convenient to describe its development here. The pituitary
body is in fact an organ derived from the epiblast of the stomodæum.
This fact has been demonstrated for Mammalia, Aves, Amphibia and
Elasmobranchii, and may be accepted as holding good for all the
Craniata[166]. The epiblast in the angle formed by the cranial flexure
becomes involuted to form the cavity of the mouth. This cavity is
bordered on its posterior surface by the front wall of the alimentary
tract, and on its anterior by the base of the fore-brain. Its
uppermost end does not at first become markedly constricted off from
the remainder, but is nevertheless the rudiment of the pituitary body.
[166] Scott states that in the larva of Petromyzon the pituitary
body is derived from the walls of the nasal pit; _Quart. J. of
Micr. Science_, Vol. XXI. p. 750. I have not myself completely
followed its development in Petromyzon, but I have observed a
slight diverticulum of the stomodæum which I believe gives origin
to it. Fuller details are in any case required before we can
admit so great a divergence from the normal development as is
indicated by Scott's statements.
Fig. 256 represents a transverse section through the head of an
Elasmobranch embryo, in which, owing to the cranial flexure, the fore
part of the head is cut longitudinally and horizontally, and the
section passes through both the fore-brain (_fb_) and the hind-brain.
Close to the base of the fore-brain are seen the mouth (_m_), and the
pituitary involution from this (_pt_). In contact with the pituitary
involution is the blind anterior termination of the throat (_al_)
which a little way back opens to the exterior by the first visceral
cleft (1 _v.c._). This figure alone suffices to demonstrate the
correctness of the above account of the pituitary body; but its truth
is still further confirmed by fig. 252; in which the mouth involution
(_pt_) is in contact with, but still separated from, the front end of
the alimentary tract. Very shortly after the septum between the mouth
and throat becomes pierced, and the two are placed in communication,
the pituitary involution becomes very partially constricted off from
the mouth involution, though still in direct communication with it. In
later stages the pituitary involution becomes longer and is dilated
terminally; while the passage connecting it with the mouth becomes
narrower and narrower, and is finally reduced to a solid cord, which
in its turn disappears.
Before the connection between the pituitary vesicle and the mouth is
obliterated the cartilaginous cranium becomes developed, and it may
then be seen that the infundibulum projects through the pituitary
space to come into close juxtaposition with the pituitary body.
After the pituitary vesicle has lost its connection with the mouth it
lies just in front of the infundibulum (figs. 250 and 255 _hph_ and
fig. 254 _pt_); and soon becomes surrounded by vascular mesoblast,
which grows in and divides it into a number of branching tubes. In
many forms the cavity of the vesicle completely disappears, and the
branches become for the most part solid [Cyclostomata and some
Mammalia (the rabbit), Elasmobranchii, Teleostei and Amphibia]. In
Reptilia, Aves and most Mammalia the lumen of the organ is more or
less retained (W. Müller, No. 344).
[FIG. 256. TRANSVERSE SECTION THROUGH THE FRONT PART OF THE HEAD OF
A YOUNG PRISTIURUS EMBRYO.
The section, owing to the cranial flexure, cuts both the fore- and
the hind-brain. It shews the premandibular and mandibular head
cavities _1pp_ and _2pp_, etc. The section is moreover somewhat
oblique from side to side.
_fb._ fore-brain; _l._ lens of eye; _m._ mouth; _pt._ upper end of
mouth, forming pituitary involution; _1ao._ mandibular aortic arch;
_1pp._ and _2pp._ first and second head cavities; _1vc._ first
visceral cleft; _V._ fifth nerve; _aun._ auditory nerve; _VII._
seventh nerve; _aa._ roots of dorsal aorta; _acv._ anterior cardinal
vein; _ch._ notochord.]
Although in the majority of the Vertebrata there is a close connection
between the pituitary body and the infundibulum, there is no actual
fusion between the two. In Mammalia the case is different. The part of
the infundibulum which lies at the hinder end of the pituitary body is
at first a simple finger-like process of the brain (fig. 255 _inf_),
but its end becomes swollen, and the lumen in this part becomes
obliterated. Its cells, originally similar to those of the other parts
of the nervous system and even (Kölliker) containing differentiated
nerve-fibres, partly atrophy, and partly assume an indifferent form,
while at the same time there grow in amongst them numerous vascular
and connective-tissue elements. The process of the infundibulum thus
metamorphosed becomes inseparably connected with the true pituitary
body, of which it is usually described as the posterior lobe. The part
of the infundibulum which undergoes this change is very probably
homologous with the saccus vasculosus of Fishes.
The true nature of the pituitary body has not yet been made out. It is
clearly a rudimentary organ in existing craniate Vertebrates, and its
development indicates that when functional it was probably a sense
organ opening into the mouth, its blind end reaching to the base of
the brain. No similar organ has as yet been found in Amphioxus, but it
seems possible perhaps to identify it with the peculiar ciliated sack
placed at the opening of the pharynx in the Tunicata, the development
of which was described at p. 18. If the suggestion is correct, the
division of the body into lobes in existing Vertebrata must be
regarded as a step towards a retrogressive metamorphosis.
Another possible view is to regard the pituitary body as a glandular
structure which originally opened into the mouth in the lower
Chordata, but which has in all existing forms ceased to be functional.
The intimate relation of the organ to the brain appears to me opposed
to this view of its nature, while on the other hand its permanent
structure is more easily explained on this view than on that
previously stated. In the Ascidians a glandular organ has been
described by Lacaze Duthiers[167] in juxtaposition to the ciliated
sack, and it is possible that this organ as well as the ciliated sack
may be related to the pituitary body. In view of this possibility
further investigations ought to be carried out in order to determine
whether the whole pituitary body is derived from the oral involution,
or whether there may not be a nervous part and a glandular part of the
organ.
[167] "Les Ascidies simples des Côtes de France." _Archives de
Biologie expér. et générale_, Vol. III. 1874, p. 329.
The Cerebral Hemispheres. It will be convenient to treat separately
the development of the cerebral hemispheres proper, and that of the
olfactory lobes.
Although the cerebral hemispheres vary more than any other part of the
brain, they are nevertheless developed from the unpaired cerebral
rudiment in a nearly similar manner throughout the series of
Vertebrata.
In the cerebral rudiment two parts may be distinguished, viz. the
floor and the roof. The former gives rise to the ganglia at the base
of the hemispheres--corpora striata, etc.--the latter to the
hemispheres proper.
[FIG. 257. DIAGRAMMATIC LONGITUDINAL HORIZONTAL SECTION THROUGH THE
FORE-BRAIN.
_3.v._ third ventricle; _lv._ lateral ventricle; _lt._ lamina
terminalis; _ce._ cerebral hemisphere; _op.th._ optic thalamus.]
The first change which takes place consists in the roof growing out
into two lobes, between which a shallow median constriction makes its
appearance (fig. 257). The two lobes thus formed are the rudiments of
the two hemispheres. The cavity of each of them opens by a widish
aperture into the vestibule at the base of the cerebral rudiment,
which again opens directly into the cavity of the third ventricle (_3
v_). The Y-shaped aperture thus formed, which leads from the cerebral
hemispheres into the third ventricle, is the foramen of Munro. The
cavity (_lv_) in each of the rudimentary hemispheres is a lateral
ventricle. The part of the cerebrum which lies between the two
hemispheres, and passes forwards from the roof of the third ventricle
round the end of the brain to the optic chiasma, is the rudiment of
the lamina terminalis (figs. 257 _lt_ and 255 _trm_). Up to this point
the development of the cerebrum is similar in all Vertebrata, but in
some forms it practically does not proceed much further.
In Elasmobranchii, although the cerebrum reaches a considerable size
(fig. 254 _cer_), and grows some way backwards over the
thalamencephalon, yet it is not in many forms divided into two
distinct lobes, but its paired nature is only marked by a shallow
constriction on the surface. The lamina terminalis in the later stages
of development grows backwards as a thick median septum which
completely separates the two lateral ventricles[168] (fig. 263).
[168] A comparison of the mode of development of this septum with
that of the septum lucidum with its contained commissures in
Mammalia clearly shews that the two structures are not
homologous, and that Miklucho-Maclay is in error in attempting to
treat them as being so.
There are, it may be mentioned, considerable variations in the
structure of the cerebrum in Elasmobranchii into which it is not
however within the scope of this work to enter.
In the Teleostei the vesicles of the cerebral hemispheres appear at
first to have a wide lumen, but it subsequently becomes almost or
quite obliterated, and the cerebral rudiment forms a small bilobed
nearly solid body. In Petromyzon (fig. 253 _ch_) the cerebral rudiment
is at first an unpaired anterior vesicle, which subsequently becomes
bilobed in the normal manner. The walls of the hemispheres become much
thickened, but the lateral ventricles persist.
In all the higher Vertebrates the division of the cerebral rudiment
into two distinct hemispheres is quite complete, and with the
deepening of the furrow between the two hemispheres the lamina
terminalis is carried backwards till it forms a thin layer bounding
the third ventricle anteriorly, while the lateral ventricles open
directly into the third ventricle.
In Amphibians the two hemispheres become united together immediately
in front of the lamina terminalis by commissural fibres, forming the
anterior commissure. They also send out anteriorly two solid
prolongations, usually spoken of as the olfactory lobes, which
subsequently fuse together.
In all Reptilia and Aves there is formed an anterior commissure, and
in the higher members of the group, especially Aves (fig. 250), the
hemispheres may obtain a considerable development. Their outer walls
are much thickened, while their inner walls become very thin; and a
well-developed ganglionic mass, equivalent to the corpus striatum, is
formed at their base.
The cerebral hemispheres undergo in Mammalia the most complicated
development. The primitive unpaired cerebral rudiment becomes, as in
lower Vertebrates, bilobed, and at the same time divided by the
ingrowth of a septum of connective tissue into two distinct
hemispheres (figs. 260 and 261 _f_ and 258 1). From this septum is
formed the falx cerebri and other parts.
The hemispheres contain at first very large cavities, communicating by
a wide foramen of Munro with the third ventricle (fig. 260). They grow
rapidly in size, and extend, _especially backwards_, and gradually
cover the thalamencephalon and the mid-brain (fig. 258 1, _f_). The
foramen of Munro becomes very much narrowed and reduced to a mere
slit.
[FIG. 258. BRAIN OF A THREE MONTHS' HUMAN EMBRYO: NATURAL SIZE.
(From Kölliker.)
1. From above with the dorsal part of hemispheres and mid-brain
removed; 2. From below. _f._ anterior part of cut wall of the
hemisphere; _f´._ cornu ammonis; _tho._ optic thalamus; _cst._
corpus striatum; _to._ optic tract; _cm._ corpora mammillaria; _p._
pons Varolii.]
The walls are originally nearly uniformly thick, but the floor becomes
thickened on each side, and gives rise to the corpus striatum (figs.
260 and 261 _st_). The corpus striatum projects upwards into each
lateral ventricle, giving to it a somewhat semilunar form, the two
horns of which constitute the permanent anterior and descending cornua
of the lateral ventricles (fig. 262 _st_).
[FIG. 259. TRANSVERSE SECTION THROUGH THE BRAIN OF A RABBIT OF FIVE
CENTIMETRES. (After Mihalkovics.)
The section passes through nearly the posterior border of the septum
lucidum, immediately in front of the foramen of Munro.
_hms._ cerebral hemispheres; _cal._ corpus callosum; _amm._ cornu
ammonis (hippocampus major); _cms._ superior commissure of the
cornua ammonis; _spt._ septum lucidum; _frx 2._ vertical fibres of
the fornix; _cma._ anterior commissure; _trm._ lamina terminalis;
_str._ corpus striatum; _ltf._ nucleus lenticularis of corpus
striatum; _vtr 1._ lateral ventricle; _vtr 3._ third ventricle;
_ipl._ slit between cerebral hemispheres.]
With the further growth of the hemisphere the corpus striatum loses
its primitive relations to the descending cornu. The reduction in size
of the foramen of Munro above mentioned is, to a large extent, caused
by the growth of the corpora striata.
[FIG. 260. TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S EMBRYO
OF 2.7 CM. IN LENGTH. (From Kölliker.)
The section passes through the level of the foramen of Munro.
_st._ corpus striatum; _m._ foramen of Munro; _t._ third ventricle;
_pl._ choroid plexus of lateral ventricle; _f._ falx cerebri; _th._
anterior part of optic thalamus; _ch._ optic chiasma; _o._ optic
nerve; _c._ fibres of the cerebral peduncles; _h._ cornu ammonis;
_p._ pharynx; _sa._ presphenoid bone; _a._ orbitosphenoid bone; _s._
points to part of the roof of the brain at the junction between the
roof of the third ventricle and the lamina terminalis; _l._ lateral
ventricle.]
The corpora striata are united at their posterior border with the
optic thalami. In the later stages of development the area of contact
between these two pairs of ganglia increases to an immense extent
(fig. 261), and the boundary between them becomes somewhat obscure, so
that the sharp distinction which exists in the embryo between the
thalamencephalon and cerebral hemispheres becomes lost. This change is
usually (Mihalkovics, Kölliker) attributed to a fusion between the
corpora striata and optic thalami, but it has recently been attributed
by Schwalbe (No. 349), with more probability, to a growth of the
original surface of contact, and an accompanying change in the
relations of the parts.
The outer wall of the hemispheres gradually thickens, while the inner
wall becomes thinner. In the latter, two curved folds, projecting
towards the interior of the lateral ventricle, become formed. These
folds extend from the foramen of Munro along nearly the whole of what
afterwards becomes the descending cornu of the lateral ventricle.
The upper fold becomes the hippocampus major (cornu ammonis) (figs.
259 _amm_, 260 and 261 _h_, and 262 _am_). When the rudiment of the
descending cornu has become transformed into a simple process of the
lateral ventricle the hippocampus major forms a prominence upon its
floor.
[FIG. 261. TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S EMBRYO
OF 2.7 CM. IN LENGTH. (From Kölliker.)
The section is taken a short distance behind the section represented
in fig. 260, and passes through the posterior part of the
hemispheres and the third ventricle.
_st._ corpus striatum; _th._ optic thalamus; _to._ optic tract; _t._
third ventricle; _d._ roof of third ventricle; _c._ fibres of
cerebral peduncles; _c´._ divergence of these fibres into the walls
of the hemispheres; _e._ lateral ventricle with choroid plexus _pl_;
_h._ cornu ammonis; _f._ primitive falx; _am._ alisphenoid; _a._
orbitosphenoid; _sa._ presphenoid; _p._ pharynx; _mk._ Meckel's
cartilage.]
The wall of the lower fold becomes very thin, and a vascular plexus,
derived from the connective-tissue septum between the hemispheres, and
similar to that of the roof of the third ventricle, is formed outside
it. It constitutes a fold projecting far into the cavity of the
lateral ventricle, and together with the vascular connective tissue in
it gives rise to the choroid plexus of the lateral ventricle (figs.
260 and 261 _pl_).
It is clear from the above description that a marginal fissure leading
into the cavity of the lateral ventricle does not exist in the sense
often implied in works on human anatomy, in that the epithelium
covering the choroid plexus, which forms the true wall of the brain,
is a continuous membrane. The _epithelium_ of the choroid plexus of
the lateral ventricle is quite independent of that of the choroid
plexus of the third ventricle, though at the foramen of Munro the roof
of the third ventricle is of course continuous with the inner wall of
the lateral ventricle (fig. 260 _s_). The _vascular elements_ of the
two plexuses form however a continuous structure.
The most characteristic parts of the Mammalian cerebrum are the
commissures connecting the two hemispheres. These commissures are (1)
the anterior commissure, (2) the fornix, and (3) the corpus callosum,
the two latter being peculiar to Mammalia.
By the fusion of the inner walls of the hemispheres in front of the
lamina terminalis a solid septum is formed, known as the septum
lucidum, continuous behind with the lamina terminalis, and below with
the corpora striata (figs. 255 and 259 _spt_). It is by a series of
differentiations within this septum that the above commissures
originate. In Man there is a closed cavity left in the septum known as
the fifth ventricle, which has however no communication with the true
ventricles of the brain.
In the septum lucidum there become first formed, below, the transverse
fibres of the anterior commissure (fig. 255 and fig. 259 _cma_), and
in the upper part the vertical fibres of the fornix (fig. 255 and fig.
259 _frx 2_). The vertical fibres meet above the foramen of Munro, and
thence diverge backwards, as the posterior pillars, to lose themselves
in the cornu ammonis (fig. 259 _amm_). Ventrally they are continued,
as the descending or anterior pillars of the fornix, into the corpus
albicans, and thence into the optic thalami.
The corpus callosum is not formed till after the anterior commissure
and fornix. It arises in the upper part of the region (septum lucidum)
formed by the fusion of the lateral walls of the hemispheres (figs.
255 and 259 _cal_), and at first only its curved anterior portion--the
genu or rostrum--is developed. This portion is alone found in
Monotremes and Marsupials. The posterior portion, which is present in
all the Monodelphia, is gradually formed as the hemispheres are
prolonged further backwards.
[FIG. 262. LATERAL VIEW OF THE BRAIN OF A CALF EMBRYO OF 5 CM.
(After Mihalkovics.)
The outer wall of the hemisphere is removed, so as to give a view of
the interior of the left lateral ventricle.
_hs._ cut wall of hemisphere; _st._ corpus striatum; _am._
hippocampus major (cornu ammonis); _d._ choroid plexus of lateral
ventricle; _fm._ foramen of Munro; _op._ optic tract; _in._
infundibulum; _mb._ mid-brain; _ch._ cerebellum; _IV.V._ roof of
fourth ventricle; _ps._ pons Varolii, close to which is the fifth
nerve with Gasserian ganglion.]
Primitively the Mammalian cerebrum, like that of the lower Vertebrata,
is quite smooth. In many of the Mammalia, _Monotremata_, _Insectivora,
etc._, this condition is nearly retained through life, while in the
majority of Mammalia a more or less complicated system of fissures is
developed on the surface. The most important, and first formed, of
these is the Sylvian fissure. It arises at the time when the
hemispheres, owing to their growth in front of and behind the corpora
striata, have assumed a somewhat bean-shaped form. At the root of the
hemispheres--the hilus of the bean--there is formed a shallow
depression, which constitutes the first trace of the Sylvian fissure.
The part of the brain lying in this fissure is known as the island of
Reil.
The olfactory lobes. The olfactory lobes, or rhinencephala, are
secondary outgrowths of the cerebral hemispheres, and contain
prolongations of the lateral ventricles, but may however be solid in
the adult state. According to Marshall they develop in Birds and
Elasmobranchs and presumably other forms later than the olfactory
nerves, so that the olfactory region of the hemispheres is indicated
before the appearance of the olfactory lobes.
In most Vertebrates the olfactory lobes arise at a fairly early stage
of development from the under and anterior part of the hemispheres
(fig. 250 _olf_). In Elasmobranchs they arise, not from the base, but
from the lateral parts of the brain (fig. 263), and become
subsequently divided into a bulbous portion and a stalk. They vary
considerably in their structure in the adult.
[FIG. 263. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.)
_ch._ cerebral hemispheres; _ol.v._ olfactory vesicle; _olf._
olfactory pit; _Sch._ Schneiderian folds; _I._ olfactory nerve. The
reference line has been accidentally taken through the nerve to the
brain; _pn._ anterior prolongation of pineal gland.]
In Amphibia the solid anterior prolongations of the cerebral
hemispheres already spoken of are usually regarded as the olfactory
lobes, but according to Götte, whose view appears to me well founded,
small papillæ, situated at the base of these prolongations, from which
olfactory nerves spring, and which contain a process of the lateral
ventricle, should properly be regarded as the olfactory lobes. These
papillæ arise prior to the solid anterior prolongations of the
hemispheres.
In Birds the olfactory lobes are small. In the chick they arise
(Marshall) on the seventh day of incubation.
_General conclusions as to the Central Nervous System._
It has been shewn above that both the brain and spinal cord are
primitively composed of a uniform wall of epithelial cells, and that
the first differentiation results in the formation of an external
layer of white matter, a middle layer of grey matter (ganglion cells),
and an inner epithelial layer. This primitive histological
arrangement, which in many parts of the brain at any rate, is only to
be observed in the early developmental stages, has a simple
phylogenetic explanation.
As has been already explained in an earlier part of this chapter the
central nervous system was originally a differentiated part of the
superficial epidermis.
This differentiation (as may be concluded from the character of the
nervous system in the Coelenterata and Echinodermata) consisted in the
conversion of the inner ends of the epithelial cells into
nerve-fibres; that is to say, that the first differentiation resulted
in the formation of a layer of white matter on the inner side of the
epidermis. The next stage was the separation of a deeper layer of the
epidermis as a layer of ganglion cells from the superficial epithelial
layer, _i.e._ the formation of a middle layer of ganglion cells and an
outer epithelial layer. Thus, phylogenetically, the same three layers
as those which first make their appearance in the ontogeny of the
vertebrate nervous system became successively differentiated, and in
both cases they are clearly placed in the same positions, because the
central canal of the vertebrate nervous system, as formed by an
involution, is at the true outer surface, and the external part of the
cord is at the true inner surface.
It is probable that a very sharp distinction between the white and
grey matter is a feature acquired in the higher Vertebrata, since in
Amphioxus there is no such sharp separation; though the nerve-fibres
are mainly situated externally and the nerve-cells internally.
As already stated in Chapter XII. the primitive division of the
nervous axis was probably not into brain and spinal cord, but into (1)
a fore-brain, representing the ganglion of the præoral lobe, and (2)
the posterior part of the nervous axis, consisting of the mid- and
hind-brains and the spinal cord. This view of the division of the
central nervous system fits in fairly satisfactorily with the facts of
development. The fore-brain is, histologically, more distinct from the
posterior part of the nervous system than the posterior parts are from
each other; the front end of the notochord forms the boundary between
these two parts of the central nervous system (_vide_ fig. 253),
ending as it does at the front termination of the floor of the
mid-brain, and finally, the nerves of the fore-brain have a different
character to those of the mid- and hind-brain.
This primitive division of the central nervous system is lost in all
the true Vertebrata, and in its place there is a secondary
division--corresponding with the secondary vertebrate head--into a
brain and spinal cord. The brain, as it is established in these forms,
is again divided into a fore-brain, a mid-brain and a hind-brain. The
fore-brain is, as we have already seen, the original ganglion of the
præoral lobe. The mid-brain appears to be the lobe, or ganglion, of
the third pair of nerves (first pair of segmental nerves), while the
hind-brain is a more complex structure, each section of which (perhaps
indicated by the constrictions which often appear at an early stage of
development) giving rise to a pair of segmental nerves is, roughly
speaking, homologous with the whole mid-brain.
The type of differentiation of each of the primitively simple vesicles
forming the fore-, the mid- and the hind-brains is very uniform
throughout the Vertebrate series, but it is highly instructive to
notice the great variations in the relative importance of the parts of
the brain in the different types. This is especially striking in the
case of the fore-brain, where the cerebral hemispheres, which on
embryological grounds we may conclude to have been hardly
differentiated as distinct parts of the fore-brain in the most
primitive types now extinct, gradually become more and more prominent,
till in the highest Mammalia they constitute a more important section
of the brain than the whole of the remaining parts put together.
The little that is known with reference to the significance of the
more or less corresponding outgrowths of the floor and roof of the
thalamencephalon, constituting the infundibulum and pineal gland, has
already been mentioned in connection with the development of these
parts.
BIBLIOGRAPHY.
(332) C. J. Carus. _Versuch einer Darstellung d. Nervensystems, etc._
Leipzig, 1814.
(333) J. L. Clark. "Researches on the development of the spinal cord
in Man, Mammalia and Birds." _Phil. Trans._, 1862.
(334) E. Dursy. "Beiträge zur Entwicklungsgeschichte des
Hirnanhanges." _Centralblatt f. d. med. Wissenschaften_, 1868. Nr. 8.
(335) E. Dursy. _Zur Entwicklungsgeschichte des Kopfes des Menschen
und der höheren Wirbelthiere._ Tübingen, 1869.
(336) A. Ecker. "Zur Entwicklungsgeschichte der Furchen und Windungen
der Grosshirn-Hemisphären im Foetus des Menschen." _Archiv f.
Anthropologie_, v. Ecker und Lindenschmidt. Vol. III. 1868.
(337) E. Ehlers. "Die Epiphyse am Gehirn d. Plagiostomen." _Zeit. f.
wiss. Zool_. Vol. XXX., suppl. 1878.
(338) P. Flechsig. _Die Leitungsbahnen im Gehirn und Rückenmark des
Menschen. Auf Grund entwicklungsgeschichtlicher Untersuchungen._
Leipzig, 1876.
(339) V. Hensen. "Zur Entwicklung des Nervensystems." _Virchow's
Archiv_, Bd. XXX. 1864.
(340) L. Löwe. "Beiträge z. Anat. u. z. Entwick. d. Nervensystems d.
Säugethiere u. d. Menschen." Berlin, 1880.
(341) L. Löwe. "Beiträge z. vergleich. Morphogenesis d. centralen
Nervensystems d. Wirbelthiere." _Mittheil. a. d. embryol. Instit.
Wien_, Vol. II. 1880.
(342) A. M. Marshall. "The Morphology of the Vertebrate Olfactory
organ." _Quart. J. of Micr. Science_, Vol. XIX. 1879.
(343) V. v. Mihalkovics. _Entwicklungsgeschichte d. Gehirns_. Leipzig,
1877.
(344) W. Müller. "Ueber Entwicklung und Bau der Hypophysis und des
Processus infundibuli cerebri." _Jenaische Zeitschrift_. Bd. VI. 1871.
(345) H. Rahl-Rückhard. "Die gegenseitigen Verhältnisse d. Chorda,
Hypophysis etc. bei Haifischembryonen, nebst Bemerkungen üb. d.
Deutung d. einzelnen Theile d. Fischgehirns." _Morphol. Jahrbuch_,
Vol. VI. 1880.
(346) H. Rathke. "Ueber die Entstehung der glandula pituitaria."
_Müller's Archiv f. Anat. und Physiol._, Bd. V. 1838.
(347) C. B. Reichert. _Der Bau des menschlichen Gehirns._ Leipzig,
1859 u. 1861.
(348) F. Schmidt. "Beiträge zur Entwicklungsgeschichte des Gehirns."
_Zeitschrift f. wiss. Zoologie_, 1862. Bd. XI.
(349) G. Schwalbe. "Beitrag z. Entwick. d. Zwischenhirns." _Sitz. d.
Jenaischen Gesell. f. Med. u. Naturwiss._ Jan. 23, 1880.
(350) Fried. Tiedemann. _Anatomie und Bildungsgeschichte des Gehirns
im Foetus des Menschen._ Nürnberg, 1816.
THE DEVELOPMENT OF THE CRANIAL AND SPINAL NERVES[169].
[169] Remak derived the posterior ganglia from the tissue of the
mesoblastic somites, and following in Remak's steps most authors
believed the peripheral nervous system to have a mesoblastic
origin. This view, which had however been rejected on theoretical
grounds by Hensen and others, was finally attacked on the ground
of observation by His (No. 297). His (No. 352, p. 458) found that
in the Fowl "the spinal ganglia of the head and trunk arose from
a small band of matter which is placed between the medullary
plate and epiblast, and the material of which he called the
'intermediate cord'." He further states that: "Before the closure
of the medullary tube this band forms a special groove--the
'intermediate groove'--placed close to the border of the
medullary plate. As the closure of the medullary plate into a
tube is completed, the earlier intermediate groove becomes a
compact cord. In the head of the embryo a longitudinal ridge
arises in this way, which separates the suture of the brain from
that of the epiblast. In the parts of the neck and in the
remaining region of the neck the intermediate cord does not lie
over the line of junction of the medullary tube, but laterally
from this and forms a ridge, triangular in section, with a slight
indrawing." This intermediate ridge gives rise to four ganglia in
the head, viz. the g. trigemini, g. acousticum, g.
glossopharyngei, and g. vagi, and in the trunk to the spinal
ganglia. In both cases it unites first with the spinal cord.
I have given in the above account, as far as possible, a literal
translation of His' own words, because the reader will thus be
enabled fairly to appreciate his meaning.
Subsequently to His' memoir (No. 297) I gave an account of some
researches of my own on this subject (No. 351), stating the whole
of the nerves to be formed as cellular outgrowths of the spinal
cord. I failed fully to appreciate that some of the stages I
spoke of had been already accurately described by His, though
interpreted by him very differently. Marshall, and afterwards
Kölliker, arrived at results in the main similar to my own, and
Hensen, independently of and nearly simultaneously with myself,
published briefly some observations on the nerves of Mammals in
harmony with my results.
His has since worked over the subject again (No. 352), and has
reaffirmed as a result of his work his original statements. I
cannot, however, accept his interpretations on the subject, and
must refer the reader who is anxious to study them more fully, to
His' own paper.
All the nerves are outgrowths of the central nervous system, but the
differences in development between the cranial and spinal nerves are
sufficiently great to make it convenient to treat them separately.
Spinal nerves. The posterior roots of the spinal nerves, as well as
certain of the cranial nerves, arise in the same manner, and from the
same structure, and are formed considerably before the anterior roots.
Elasmobranch fishes may be taken as the type to illustrate the mode of
formation of the spinal nerves.
The whole of the nerves in question arise as outgrowths of a median
ridge of cells, which makes its appearance on the dorsal side of the
spinal cord (fig. 264 A, _pr_). This ridge has been called by Marshall
the neural crest. At each point, where a pair of nerves will be
formed, two pear-shaped outgrowths project from it, one on each side;
and apply themselves closely to the walls of the spinal cord (fig. 264
B, _pr_). These outgrowths are the rudiments of the posterior nerves.
While still remaining attached to the dorsal summit of the neural cord
they grow to a considerable size (fig. 264 B, _pr_).
[FIG. 264 A. TRANSVERSE SECTION THROUGH A PRISTIURUS EMBRYO SHEWING
THE PROLIFERATION OF CELLS TO FORM THE NEURAL CREST.
_pr._ neural crest; _nc._ neural canal; _ch._ notochord; _ao._
aorta.]
[FIG. 264 B. TRANSVERSE SECTION THROUGH THE TRUNK OF AN EMBRYO
SLIGHTLY OLDER THAN FIG. 28 E.
_nc._ neural canal; _pr._ posterior root of spinal nerve; _x._
subnotochordal rod; _ao._ aorta; _sc._ somatic mesoblast; _sp._
splanchnic mesoblast; _mp._ muscle-plate; _mp´._ portion of
muscle-plate converted into muscle; _Vv._ portion of the vertebral
plate which will give rise to the vertebral bodies; _al._ alimentary
tract.]
[FIG. 265. VERTICAL LONGITUDINAL SECTION THROUGH PART OF THE TRUNK
OF A YOUNG SCYLLIUM EMBRYO.
_com._ commissure uniting the dorsal ends of the posterior
nerve-roots; _pr._ ganglia of posterior roots; _ar._ anterior roots;
_st._ segmental tubes; _sd._ segmental duct; _g.e._ epithelium
lining the body cavity in the region of the future germinal ridge.]
The attachment to the dorsal summit is not permanent, but before
describing the further fate of the nerve-rudiments it is necessary to
say a few words as to the neural crest. At the period when the nerves
have begun to shift their attachment to the spinal cord, there makes
its appearance, in Elasmobranchii, a longitudinal commissure
connecting the dorsal ends of all the spinal nerves (figs. 265, 266
_com_), as well as those of the vagus and glossopharyngeal nerves.
This commissure has as yet only been found in a complete form in
Elasmobranchii; but it is nevertheless to be regarded as a very
important morphological structure.
[FIG. 266. SPINAL NERVES OF SCYLLIUM IN LONGITUDINAL SECTION TO SHEW
THE COMMISSURE CONNECTING THEM.
A. Section through a series of nerves.
B. Highly magnified view of the dorsal part of a single nerve, and
of the commissure connected with it.
_com._ commissure; _sp.g._ ganglion of posterior root; _ar._
anterior root.]
It is probable, though the point has not yet been definitely made out,
that this commissure is derived from the neural crest, which appears
therefore to separate into two cords, one connected with each set of
dorsal roots.
[FIG. 267. SECTION THROUGH THE DORSAL PART OF THE TRUNK OF A TORPEDO
EMBRYO.
_pr._ posterior root of spinal nerve; _g._ spinal ganglion; _n._
nerve; _ar._ anterior root of spinal nerve; _ch._ notochord; _nc._
neural canal; _mp._ muscle-plate.]
Returning to the original attachment of the nerve-rudiments to the
medullary wall, it has been already stated that this attachment is not
permanent. It becomes, in fact, at about the time of the appearance of
the above commissure, either extremely delicate or absolutely
interrupted.
The nerve-rudiment now becomes divided into three parts (figs. 267 and
268), (1) a proximal rounded portion, to which is attached the
longitudinal commissure (_pr_); (2) an enlarged portion, forming the
rudiment of a ganglion (_g_ and _sp g_); (3) a distal portion, forming
the commencement of the nerve (_n_). The proximal portion may very
soon be observed to be united with the side of the spinal cord at a
very considerable distance from its original point of attachment.
Moreover the proximal portion of the nerve is attached, not by its
extremity, but by its side, to the spinal cord (fig. 268 _x_). The
dorsal extremities of the posterior roots are therefore free.
This attachment of the posterior nerve-root to the spinal cord is, on
account of its small size, very difficult to observe. In favourable
specimens there may however be seen a distinct cellular prominence
from the spinal cord, which becomes continuous with a small prominence
on the lateral border of the nerve-root near its proximal extremity.
The proximal extremity of the nerve is composed of cells, which, by
their small size and circular form, are easily distinguished from
those which form the succeeding or ganglionic portion of the nerve.
This part has a swollen configuration, and is composed of large
elongated cells with oval nuclei. The remainder of the rudiment forms
the commencement of the true nerve. This also is, at first, composed
of elongated cells[170].
[170] The cellular structure of embryonic nerves is a point on
which I should have anticipated that a difference of opinion was
impossible, had it not been for the fact that His and Kölliker,
following Remak and other older embryologists, absolutely deny
the fact. I feel quite sure that no one studying the development
of the nerves in Elasmobranchii with well-preserved specimens
could for a moment be doubtful on this point, and I can only
explain His' denial on the supposition that his specimens were
utterly unsuited to the investigation of the nerves. I do not
propose in this work entering into the histogenesis of nerves,
but may say that for the earlier stages of their growth, at any
rate, my observations have led me in many respects to the same
results as Götte (_Entwick. d. Unke_, pp. 482-483), except
that I hold that adequate proof is supplied by my investigations
to demonstrate that the nerves are for their whole length
originally formed as outgrowths of the central nervous system. As
the nerve-fibres become differentiated from the primitive
spindle-shaped cells, the nuclei become relatively more sparse,
and this fact has probably misled Kölliker. Löwe, while admitting
the existence of nuclei in the nerves, states that they belong to
mesoblastic cells which have wandered into the nerves. This is a
purely gratuitous assumption, not supported by observation of the
development.
It is extremely difficult to decide whether the permanent attachment
of the posterior nerve-roots to the spinal cord is entirely a new
formation, or merely due to the shifting of the original point of
attachment. I am inclined to adopt the former view, which is also held
by Marshall and His, but may refer to fig. 269, shewing the roots
after they have become attached to the side, as distinct evidence in
favour of the view that the attachment simply becomes shifted, a
process which might perhaps be explained by a growth of the dorsal
part of the spinal cord. The change of position in the case of some of
the cranial nerves is, however, so great that I do not think that it
is possible to account for it without admitting the formation of a new
attachment.
[FIG. 268. SECTION THROUGH THE DORSAL REGION OF A PRISTIURUS EMBRYO.
_pr._ posterior root; _sp.g._ spinal ganglion; _n._ nerve; _x._
attachment of ganglion to spinal cord; _nc._ neural canal; _mp._
muscle-plate; _ch._ notochord; _i._ investment of spinal cord.]
The anterior roots of the spinal nerves appear somewhat later than the
posterior roots, but while the latter are still quite small. Each of
them (fig. 269 _ar_) arises as a small but distinct conical outgrowth
from a ventral corner of the spinal cord, before the latter has
acquired its covering of white matter. From the very first the
rudiments of the anterior roots have a somewhat fibrous appearance and
an indistinct form of peripheral termination, while the protoplasm of
which they are composed becomes attenuated towards its end. They
differ from the posterior roots in never shifting their point of
attachment to the spinal cord, in not being united with each other by
a commissure, and in never developing a ganglion.
The anterior roots grow rapidly, and soon form elongated cords of
spindle-shaped cells with wide attachments to the spinal cord (fig.
267). At first they pass obliquely and nearly horizontally outwards,
but, before reaching the muscle-plates, they take a bend downwards.
One feature of some interest with reference to the anterior roots is
the fact that they arise not vertically below, but alternately with
the posterior roots: a condition which persists in the adult. They are
at first quite separate from the posterior roots; but about the stage
represented in fig. 267 a junction is effected between each posterior
root and the corresponding anterior root. The anterior root joins the
posterior at some little distance below its ganglion (figs. 265 and
266).
Although I have made some efforts to determine the eventual fate of
the commissure uniting the dorsal roots, I have not hitherto met with
success. It grows thinner and thinner, becoming at the same time
composed of fibrous protoplasm with imbedded nuclei, and finally
ceases to be recognisable. I can only conclude that it gradually
atrophies, and ultimately vanishes.
After the junction of the posterior and anterior roots the compound
nerve extends downwards, and may easily be traced for a considerable
distance. A special dorsal branch is given off from the ganglion on
the posterior root (fig. 275 _dn_). According to Löwe the fibres of
the anterior and posterior roots can easily be distinguished in the
higher types by their structure and behaviour towards colouring
reagents, and can be separately traced in the compound nerve.
[FIG. 269. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF A YOUNG
TORPEDO EMBRYO TO SHEW THE ORIGIN OF THE ANTERIOR AND POSTERIOR
ROOTS OF THE SPINAL NERVES.
_pr._ posterior root of spinal nerve; _ar._ anterior root of spinal
nerve; _mp._ muscle-plate; _ch._ notochord; _vr._ mesoblast cells
which will form the vertebral bodies.]
So far as has been made out, the development of the spinal nerves of
other Vertebrates agrees in the main with that in Elasmobranchii, but
_no dorsal commissure has yet been discovered_, except in the case of
the first two or three spinal nerves of the Chick.
In the Chick (Marshall, No. 353) the posterior roots, during their
early stages, closely resemble those in Elasmobranchii, though their
relatively smaller size makes them difficult to observe. They at first
extend more or less horizontally outwards above the muscle-plates (as
a few of the nerves also do to some extent in Elasmobranchii), but
subsequently lie close to the sides of the neural canal. They are
shewn in this position in fig. 116 _sp.g._ There does not appear to be
a continuous crest connecting the roots of the posterior nerves. The
later stages of the development are precisely like those in
Elasmobranchii.
The anterior roots have not been so satisfactorily investigated as the
posterior, but they grow out, possibly by several roots for each
nerve, from the ventral corners of the spinal cord, and subsequently
become attached to the posterior nerves.
I have observed the development of the posterior roots in Lepidosteus,
in which they appear as projections from the dorsal angles of the
spinal cord, extending laterally outwards and, at first, having their
extremities placed dorsally to the muscle-plates.
[FIG. 270. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD
OF AN EMBRYO CHICK OF THIRTY HOURS.
_hb._ hind-brain; _vg._ vagus nerve; _ep._ epiblast; _ch._
notochord; _x._ thickening of hypoblast (possibly a rudiment of the
subnotochordal rod); _al._ throat; _ht._ heart; _pp._ body cavity;
_so._ somatic mesoblast; _sf._ splanchnic mesoblast; _hy._
hypoblast.]
The cranial nerves[171]. The earliest stages in the development of the
cranial nerves have been most satisfactorily studied, especially by
Marshall (No. 354), in the Chick, while the later stages have been
more fully worked out in Elasmobranchii, where, moreover, they present
a very primitive arrangement. In the Chick certain of the cranial
nerves arise before the complete closure of the neural groove. These
nerves are formed as paired outgrowths of a continuous band composed
of two laminæ, connecting the dorsal end of the incompletely closed
medullary canal with the external epiblast. This mode of development
will best be understood by an examination of fig. 270, where the two
roots of the vagus nerve (_vg_) are shewn growing out from the neural
band. Shortly after this stage the neural band, becoming separated
from the epiblast, constitutes a crest attached to the roof of the
brain, while its two laminæ become fused. The relation of the cranial
nerves to the brain then becomes exactly the same as that of the
posterior roots of the spinal nerves to the spinal cord.
[171] The optic nerves are for obvious reasons dealt with in
connection with the development of the eye.
It does not appear possible to decide whether the mode of development
of the cranial nerves in the Chick, or that of the posterior roots of
the spinal nerves, is the more primitive. The difference in
development between the two sets of nerves probably depends upon the
relative time of the closure of the neural canal. The neural crest
clearly belongs to the brain, from the fact of its remaining connected
with the latter when the medullary tube separates from the external
epiblast.
It is not known whether the cranial nerves originate before the
closure of the neural canal in other forms besides the Chick.
The neural crest of the brain is continuous with that of the spinal
cord, and on its separation from the central nervous axis forms on
each side a commissure, uniting the posterior cranial nerves with the
spinal nerves, and continuous with the commissure connecting together
the latter nerves.
Anteriorly, the neural crest extends as far as the roof of the
mid-brain[172]. The pairs of nerves which undoubtedly grow out from it
are the third pair (Marshall), the fifth, the seventh and auditory (as
a single root), the glossopharyngeal, and the various elements of the
vagus (as separate roots in Elasmobranchii, but as a single root in
Aves). Marshall holds that the olfactory nerve probably also
originates from this crest. It will however be convenient to deal
separately with this nerve, after treating of the other nerves which
undoubtedly arise from the neural crest.
[172] Marshall holds that the neural crest extends in front of
the region of the optic vesicle. I have been unable completely to
satisfy myself of the correctness of this statement. In my
specimens the epiblast along the line of infolding of this part
of the roof of the brain is much thickened, but what Marshall
represents as a pair of outgrowths from it like those of a true
nerve (No. 354, Pl. II. fig. 6) appears to me in my specimens to
be part of the external epiblast; and I believe that they remain
connected with the external epiblast on the complete separation
of the brain from it.
The cranial nerves just enumerated present in their further
development many points of similarity; and the glossopharyngeal nerve,
as it develops in Elasmobranchii, may perhaps be taken as typical.
This nerve is connected by a commissure with those behind, but this
fact may for the moment be left out of consideration. Springing at
first from the dorsal line of the hind-brain immediately behind the
level of the auditory capsule, it apparently loses this primitive
attachment and acquires a secondary attachment about halfway down the
side of the hind-brain. The primitive undifferentiated rudiment soon
becomes divided, exactly like a true posterior root of a spinal nerve,
into a root, a ganglion and a nerve. The main branch of the nerve
passes ventralwards, and supplies the first branchial arch (fig. 271
_gl_). Shortly afterwards it sends forwards a smaller branch, which
passes to the hyoid arch in front; so that the nerve forks over the
hyobranchial cleft. A typical cranial nerve appears therefore, except
as concerns its relations to the clefts, to develop precisely like the
posterior root of the spinal nerve.
Most of the cranial nerves of the above group, in correlation with the
highly differentiated character of the head, acquire secondary
differentiations, and render necessary a brief description of what is
known with reference to their individual development.
The Glossopharyngeal and Vagus Nerves. Behind the ear there are
formed, in Scyllium, a series of five nerves which pass down to
respectively the first, second, third, fourth and fifth branchial
arches.
For each arch there is thus one nerve, whose course lies close to the
posterior margin of the preceding cleft; a second anterior branch,
forking over the cleft and passing to the arch in front, being
developed later. These nerves are connected with the brain by roots at
first attached to the dorsal summit, but eventually situated about
halfway down the sides. The foremost of them is the glossopharyngeal.
The next four are, as has been shewn by Gegenbaur[173], equivalent to
four independent nerves, but form together a compound nerve, which we
may briefly call the vagus.
[173] "Ueber d. Kopfnerven von Hexanchus," etc., _Jenaische
Zeitschrift_, Vol. VI. 1871.
[FIG. 271. VIEWS OF THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES
AS TRANSPARENT OBJECTS.
A. Pristiurus embryo of the same stage as fig. 28 F.
B. Somewhat older Scyllium embryo.
_III._ third nerve; _V._ fifth nerve; _VII._ seventh nerve; _au.n._
auditory nerve; _gl._ glossopharyngeal nerve; _Vg._ vagus nerve;
_fb._ fore-brain; _pn._ pineal gland; _mb._ mid-brain; _hb._
hind-brain; _iv.v._ fourth ventricle; _cb._ cerebellum; _ol._
olfactory pit; _op._ eye; _au.V._ auditory vesicle; _m._ mesoblast
at base of brain; _ch._ notochord; _ht._ heart; _Vc._ visceral
clefts; _eg._ external gills; _pp._ sections of body cavity in the
head.]
This compound nerve together with the glossopharyngeal soon attains a
very complicated structure, and presents several remarkable features.
There are present five branches (fig. 271 B), viz. the glossopharyngeal
(_gl_) and four branches of the vagus, the latter probably arising by
a considerably greater number of strands from the brain[174]. All the
strands from the brain are united together by a thin commissure (fig.
271 B, _vg_), continuous with the commissure of the posterior roots of
the spinal nerves, and from this commissure the five branches are
continued obliquely ventralwards and backwards, and _each of them
dilates into a ganglionic swelling_. They all become again united
together by a second thick commissure, which is continued backwards as
the intestinal branch of the vagus nerve. The nerves, however, are
continued ventralwards each to its respective arch. From the lower
commissure springs the lateral nerve, at a point whose relations to
the branches of the vagus I have not certainly determined.
[174] "Ueber d. Kopfnerven von Hexanchus," etc., _Jenaische
Zeitschrift_, Vol. VI. 1871.
With reference to the dorsal commissure, which is almost certainly
derived from the original neural crest, it is to be noted that there
is a longish stretch of it between the last branch of the vagus and
the first spinal nerve, which is probably the remains of a part of the
commissure which connected the posterior branches of the vagus, at a
stage in the evolution of the Vertebrata, when the posterior visceral
clefts were still present. These branches of the vagus are probably
partially preserved in the ramifications of the intestinal stem of the
vagus (Gegenbaur). The origin of the ventral commissure, continued as
the intestinal branch of the vagus, has not been embryologically
worked out.
The lateral nerve may very probably be a dorsal sensory branch of the
vagus, whose extension into the posterior part of the trunk has been
due to the gradual backward elongation of the lateral line[175],
causing the nerve supplying it to elongate at the same time (_vide_
Section on lateral line).
[175] The peculiar distribution of branches of the fifth and
seventh nerves to the lateral line, which is not uncommon, is to
be explained in the same manner.
In the Chick the common rudiment for the vagus and glossopharyngeal
nerves (Marshall), which has already been spoken of, subsequently
divides into two parts, an anterior forming the glossopharyngeal
nerve, and a posterior forming the vagus nerve.
The seventh and auditory nerves. As shewn by Marshall's and my own
observations there is a common rudiment for the seventh and auditory
nerves. This rudiment divides almost at once into two branches. The
anterior of these pursues a straight course to the hyoid arch (fig.
271 A, _VII._) and forms the rudiment of the facial nerve; the second
of the two (fig. 271 A, _au.n_), which is the rudiment of the auditory
nerve, develops a ganglionic enlargement and, turning backwards,
closely hugs the ventral wall of the auditory involution (fig. 272).
The seventh or facial nerve soon becomes more complicated. It early
develops, like the glossopharyngeal and vagus nerves, a branch, which
forks over the cleft in front (spiracle), and supplies the mandibular
arch (fig. 271 B). This branch forms the præspiracular nerve of the
adult, and is homologous with the chorda tympani of Mammalia. Besides
however giving rise to this typical branch it gives origin, at a very
early period, to two other rather remarkable branches; one of these,
arising from its dorsal anterior border, passes forwards to the _front
part of the head_, immediately dorsal to the ophthalmic branch of the
fifth to be described directly. This nerve is the portio major or
superficialis of the nerve usually known as the ramus ophthalmicus
superficialis in the adult[176].
[176] The two branches of the ramus ophthalmicus superficialis
were spoken of as the ram. opth. superficialis and ram. opth.
profundus in my _Monograph on Elasmobranch Fishes_. The
nomenclature in the text is Schwalbe's, which is probably more
correct than mine.
The other branch of the seventh is the palatine branch--superficial
petrosal of Mammalia--the course of which has been more fully
investigated by Marshall than by myself. He has shewn that it arises
"just below the root of the ophthalmic branch," and "runs downwards
and forwards, lying parallel and immediately superficial to the
maxillary branch of the fifth nerve." This branch of the seventh nerve
appears to bear the same sort of relation to the superior maxillary
branch of the fifth nerve, that the ophthalmic branch of the seventh
does to the ophthalmic branch of the fifth.
Both the root of the seventh and its main branches are gangliated.
The auditory nerve is probably to be regarded as a specially
differentiated part of a dorsal branch of the seventh, while the
ophthalmic branch may not improbably be a dorsal branch comparable to
a dorsal branch of one of the spinal nerves.
The fifth nerve. Shortly after its development the root of the fifth
nerve shifts so as to be attached about halfway down the side of the
brain. A large ganglion becomes developed close to the root, which
forms the rudiment of the Gasserian ganglion. The main branch of the
nerve grows into the mandibular arch (fig. 271 A, _V_), maintaining
towards it similar relations to those of the posterior nerves to their
respective arches.
Two other branches very soon become developed, which were not properly
distinguished in my original account. The dorsal one takes a course
parallel to the ophthalmic branch of the seventh nerve, and forms,
according to the nomenclature already adopted, the portio profunda of
the ophthalmicus superficialis of the adult.
The second nerve (fig. 271 A) passes forwards, above the mandibular
head cavity, and is directed straight towards the eye, _near which it
meets and unites with the third nerve_, where the ciliary ganglion is
developed (Marshall). This branch is usually called the ophthalmic
branch of the fifth nerve, but Marshall rightly prefers to call it the
communicating branch between the fifth and third nerves[177].
[177] Marshall thinks that this nerve may be the remains of the
commissure originally connecting the roots of the third and fifth
nerves. This suggestion can only be tested by further
observations.
Later than these two branches there is developed a third branch,
passing to the front of the mouth, and forming the superior maxillary
branch of the adult (fig. 271 B).
Of the branches of the fifth nerve the main mandibular branch is
obviously comparable to the main branch of the posterior nerves. The
superficial ophthalmic branch is clearly equivalent to the ophthalmic
branch of the seventh. The superior maxillary is usually held to be
equivalent to that branch of the posterior nerves which forms the
anterior limb of the fork over a cleft. The similarity between the
course of this nerve and that of the palatine branch of the seventh,
resembling as it does the similar course of the ophthalmic branches of
the two nerves, suggests that it may perhaps really be the homologue
of the palatine branch of the seventh, there being no homologue of the
typical anterior branch of the other cranial nerves.
The third nerve. Our knowledge of the development of the third nerve
is entirely due to Marshall. He has shewn that in the Chick there is
developed from the neural crest, on the roof of the mid-brain, an
outgrowth on each side, very similar to the rudiment of the posterior
nerves. This outgrowth, the presence of which I can confirm, he
believes to be the third nerve, but although he is probably right in
this view, it must be borne in mind that there is no direct evidence
on the point, the fate of the outgrowth in question not having been
satisfactorily followed.
At a very considerably later period a nerve may be found _springing
from the floor of the mid-brain_, which is undoubtedly the third
nerve, and which Marshall supposes to be the above rudiment, which has
shifted its position. It is shewn in Scyllium in fig. 271 B, _III._ A
few intermediate stages between this and the earliest condition of the
nerve have been imperfectly traced by Marshall.
The nerve at the stage represented in fig. 271 B arises from a
ganglionic root, and "runs as a long slender stem almost horizontally
backwards, then turns slightly outwards to reach the interval between
the dorsal ends of the first and second head cavities, where it
expands into a small ganglion." This ganglion, as first suggested by
Schwalbe (No. 357), and subsequently proved embryologically by
Marshall, is the ciliary ganglion. From the ciliary ganglion two
branches arise; one branch continuing the main stem of the nerve, and
obviously homologous with the main branch of the other nerves, and the
other passing directly forwards "along the top of the first head
cavity, then along the inner side of the eye, and finally terminating
at the anterior extremity of the head, just dorsal of the olfactory
pit."
The partial separation, in many forms, of the ciliary ganglion from
the stem of the third nerve has led to the erroneous view (disproved
by the researches of Marshall and Schwalbe) that the ciliary ganglion
belongs to the fifth nerve. The connecting branch of the fifth nerve
often becomes directly continuous with the anterior branch of the
third nerve, and the two together probably constitute the nerve known
as the ramus ophthalmicus profundus (Marshall). Further embryological
investigations will be required to shew whether this nerve is
homologous with the nasal branch of the fifth nerve in Mammalia.
Relations of the nerves to the head-cavities. The cranial nerves,
whose development has just been given, bear certain very definite
relations to the mesoblastic structures in the head, of the nature of
somites, which are known as the head-cavities. Each cranial nerve is
typically placed immediately behind the head-cavity of its somite.
Thus the main branch of the fifth nerve lies in contact with the
posterior wall of the mandibular cavity, as shewn in section in fig.
272 _V. 2pp_ and in surface view in fig. 271; the main branch of the
seventh nerve occupies a similar position in relation to the hyoid
cavity; and, as Marshall has recently shewn, the main branch of the
third nerve adjoins the posterior border of the front cavity,
described by me as the premandibular cavity. Owing to the early
conversion of the walls of the posterior head-cavities into muscles,
their relations to the nerves are not quite so clear as in the case of
the anterior cavities, though, as far as is known, they are precisely
the same.
[FIG. 272. TRANSVERSE SECTION THROUGH THE FRONT PART OF THE HEAD OF
A YOUNG PRISTIURUS EMBRYO.
The section, owing to the cranial flexure, cuts both the fore- and
the hind-brain. It shews the præmandibular and mandibular
head-cavities _1pp_ and _2pp_, etc.
_fb._ fore-brain; _l._ lens of eye; _m._ mouth; _pt._ upper end of
mouth, forming pituitary involution; _1ao._ mandibular aortic arch;
_1pp._ and _2pp._ first and second head-cavities; _1vc._ first
visceral cleft; _V._ fifth nerve; _aun._ ganglion of auditory nerve;
_VII._ seventh nerve; _aa._ dorsal aorta; _acv._ anterior cardinal
vein; _ch._ notochord.]
_Anterior nerve-roots in the brain._
During my investigations on the development of the cranial nerves I
was unable to find any roots comparable with the anterior roots of the
spinal nerves, and propounded an hypothesis (suggested by the absence
of anterior spinal roots in Amphioxus[178]) that the head and trunk
had become differentiated from each other at a stage when mixed motor
and sensory posterior roots were the only roots present, and I
supposed the cranial and spinal nerves to have been _independently_
evolved from a common ground form, the resulting types of nerves being
so different that no roots strictly comparable with the anterior roots
of spinal nerves were to be found in the cranial nerves.
[178] Schneider holds that anterior roots are present in
Amphioxus, but I have been unable to satisfy myself of their
presence.
The views put forward by me on this subject, though accepted by
Schwalbe (No. 357), have in other quarters not met with much favour.
Wiedersheim holds that it is impossible to believe that the cranial
nerves are simpler than the spinal nerves. Such simplicity, which is
clearly not found, I have never asserted to exist; I have only stated
that the cranial nerves, in acquiring the complicated character they
have in the adult, do not develop anterior roots comparable with those
of the spinal nerves. Marshall also strongly objects to my views, and
has made some observations for the purpose of testing them, leading to
some very interesting results, which I proceed to state, and I will
then explain my opinion concerning them.
The most important observation of Marshall on this subject concerns
the sixth nerve. In both the Chick and Scyllium he has detected a
nerve (the first development of which has unfortunately not been made
out) arising by a series of roots from the base of the hind-brain. By
tracing this nerve to the external rectus muscle of the eye he has
satisfactorily identified it as the sixth nerve. "_Neither in the
nerve nor in its roots are there any ganglion cells._" This nerve he
finds to be placed vertically below the roots of the seventh nerve;
and it is not visible till much later than the cranial nerves above
described.
In addition to this nerve Marshall has found, both in the third nerve
and in the fifth nerve, a series of non-gangliated roots, which arise
in a manner not yet satisfactorily elucidated, considerably later
than, and in front of, the main roots. These roots join the gangliated
roots on the proximal side of the ganglion or in the ganglion[179];
and Marshall believes them to be homologous with the anterior roots of
spinal nerves, while he holds the sixth nerve to be an anterior root
of the seventh nerve.
[179] These non-gangliated roots of the fifth nerve are not to be
confounded with the motor root of the fifth nerve in higher
types. They appear to form the anterior root of the adult which
gives origin to the ramus ophthalmicus.
In addition to these nerves Marshall holds certain ventral roots,
which occur in Elasmobranchs close to the boundary of the spinal cord
and medulla, and which probably form the hypoglossal nerve of higher
types, to be anterior roots of the vagus. It is very difficult to
prove anything definitely about these nerves, but, for reasons stated
in my work on _Elasmobranch Fishes_, I am inclined to regard them as
anterior roots of one or more spinal nerves.
Before attempting to decide how far Marshall's views about the
so-called anterior roots of the seventh, the fifth and the third
nerves are well founded it will conduce to clearness to state the
characters and relations of the two roots of spinal nerves.
The posterior root is (1) always _purely sensory_; (2) it always
develops a ganglion. The anterior root is (1) always purely motor; (2)
it always joins the posterior root _below_ the ganglion, except in
Petromyzon (though not in Myxine) where the two roots are stated to be
independent.
How far do Marshall's anterior and posterior roots of the cranial
nerves exhibit these respective peculiarities?
With reference to the sixth and seventh nerves he states "we must
regard the sixth nerve as having the same relation to the seventh that
the anterior root of a spinal nerve has to the posterior root." On
this I would remark (1) that the posterior root of this nerve is a
mixed sensory and motor nerve and therefore differs in a very
fundamental point from that of a spinal nerve; (2) the sixth nerve
though resembling the anterior root of a spinal nerve in being motor
and without a ganglion, differs from the nearly universal arrangement
of spinal nerves in not uniting with the seventh.
With reference to the fifth nerve it is to be observed that it is by
no means certain that the whole of the motor fibres are supplied by
the so-called anterior roots, and that these roots differ again in the
most marked manner from the anterior roots of spinal nerves in joining
the main root of the nerve _above_ (nearer the brain), and not as in a
spinal nerve _below_ the ganglion. The gangliated root of the third
nerve is purely motor[180], and its so-called anterior roots again
differ from the anterior roots of spinal nerves, in the same manner as
those of the fifth nerve.
[180] If Marshall's view about the ramus ophthalmicus profundus
(p. 461) is correct, the third must still be, as it no doubt was
primitively, a mixed motor and sensory nerve.
With reference to the glossopharyngeal and vagus nerves I would merely
remark that no anterior root has even been suggested for the
glossopharyngeal nerve and that the posterior roots of both these
nerves contain a mixture of sensory and motor fibres.
In view of these facts, my original hypothesis appears to me to be
confirmed by Marshall's observations.
The fact of all the posterior roots of the above cranial nerves
(except the third which may be purely motor) being mixed motor and
sensory roots appears to me to demonstrate that the starting-point of
their differentiation was a mixed nerve with a single dorsal root; and
that they did not therefore become differentiated from nerves built on
the same type as the spinal nerves with dorsal sensory and ventral
motor roots. The presence of such non-gangliated roots as those of the
third and fifth nerves is not a difficulty to this view. Considering
that the cranial nerves are more highly differentiated than the spinal
nerves, and have more complicated functions to perform, it would be
surprising if there had not been developed nonganglionated roots
_analogous to_, but not of course homologous with, the anterior roots
of the spinal nerves[181].
[181] In the higher types, as is well known, the fifth nerve has
its roots formed on the same type as a spinal nerve. The fact
that this is not the case in the lower types, either in the
embryo or the adult, is a clear indication, to my mind, that the
mammalian arrangement of the roots of the fifth nerve has been
secondarily acquired, a fact which is a most striking
confirmation of my views as to the differences between the
cranial and spinal nerves.]
As to the sixth nerve further embryological investigations are
requisite before its true position in the series can be determined;
but it appears to me very probable that it is a product of the
differentiation of the seventh nerve.
The fourth nerve. No embryological investigations have been made with
reference to the fourth nerve. It is possible that it is a segmental
nerve comparable with the third nerve, and that the only remnant still
left of the segment to which it belongs is the superior oblique muscle
of the eye. If this is the case there must have been two præmandibular
segments, viz. that belonging to the third nerve, and that belonging
to the fourth nerve. Against this view of the fourth nerve is the
fact, urged with great force by Marshall, that the superior oblique
muscle is in front of the other eye muscles, and that the fourth nerve
therefore crosses the third nerve to reach its destination.
The Olfactory nerve. It was shewn in my monograph on Elasmobranch
Fishes that the olfactory nerve grew out from the brain in the same
manner as other nerves; and Marshall (No. 355), to whom we are
indebted for the greater part of our knowledge on the development of
this nerve, has proved that it arises prior to the differentiation of
the olfactory lobes.
The earliest stages in the development of the nerve have not been made
out. Marshall, as already stated, finds that in the Chick the neural
crest is continued in front of the optic vesicles, and holds that this
fact is strong _a priori_ evidence in favour of the nerve growing out
from it. As mentioned above, note on p. 456, I cannot without further
evidence accept Marshall's statements on this point. In any case
Marshall has not yet been able again to find an olfactory nerve till
long after the disappearance of the neural crest. The olfactory nerve
at the next stage observed forms an outgrowth of fusiform cells
springing on either side from near the summit of the fore-brain; and
at fifty hours it ends close to a slight thickening of the epiblast
forming the first rudiment of the olfactory pit, with the walls of
which it soon becomes united.
[FIG. 273. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.)
_c.h._ cerebral hemispheres; _ol.v._ olfactory vesicle; _olf._
olfactory pit; _Sch._ Schneiderian folds; _I._ olfactory nerve. The
reference line has been accidentally taken through the nerve to the
brain; _pn._ pineal gland.]
The growth of the cerebral hemispheres causes its point of insertion
in the brain to be relatively shifted; and on the development of the
olfactory lobes (_vide_ pp. 444, 445) it arises from them (fig. 273).
In Elasmobranchs there is a large development of ganglion cells near
its root. From Marshall's figures these appear also to be present in
the Chick, but they do not seem to have been found in other forms. In
both Teleostei and Amphibia the olfactory nerves are at first
extremely short.
Marshall holds that the olfactory nerve is a segmental nerve
equivalent to the third, fifth, seventh etc. nerves. It has been
already stated that in my opinion the origin of the olfactory nerves
from the fore-brain, which I hold to be the ganglion of the præoral
lobe, negatives this view. The mere fact of these nerves originating
as an outgrowth from the central nervous system is no argument in
favour of Marshall's view of their nature; and even if Marshall's
opinion that they arise from the neural crest should turn out to be
well founded, this fact would not prove their segmental nature,
because their origin from this crest would, as indicated in the next
paragraph, merely seem to imply that they primitively arose from the
lateral borders of the nerve-plate from which the cerebrospinal tube
has been formed.
Situation of the dorsal roots of the cranial and spinal nerves. The
probable explanation of the origin of nerves from the neural crest has
already been briefly given (p. 316). It is that the neural crest
represents the original lateral borders of the nervous plate, and
that, in the mechanical folding of the nervous plate to form the
cerebrospinal canal, its two lateral borders have become approximated
in the median dorsal line to form the neural crest. The subsequent
shifting of the nerves I am unable to explain, and the meaning of the
transient longitudinal commissure connecting the nerves is also
unknown. The folding of the neural plate must have extended to the
region of the origin of the olfactory nerves, so that, as just stated,
there would be no special probability of the olfactory nerves
belonging to the same category as the other dorsal nerves from the
fact of their springing from the neural crest.
BIBLIOGRAPHY OF THE PERIPHERAL NERVOUS SYSTEM.
(351) F. M. Balfour. "On the development of the spinal nerves in
Elasmobranch Fishes." _Philosophical Transactions_, Vol. CLXVI. 1876;
_vide_ also, _A monograph on the development of Elasmobranch Fishes_.
London, 1878, pp. 191-216.
(352) W. His. "Ueb. d. Anfänge d. peripherischen Nervensystems."
_Archiv f. Anat. u. Physiol._, 1879.
(353) A. M. Marshall. "On the early stages of development of the
nerves in Birds." _Journal of Anat. and Phys._, Vol. XI. 1877.
(354) A. M. Marshall. "The development of the cranial nerves in the
Chick." _Quart. J. of Micr. Science_, Vol. XVIII. 1878.
(355) A. M. Marshall. "The morphology of the vertebrate olfactory
organ." _Quart. J. of Micr. Science_, Vol. XIX. 1879.
(356) A. M. Marshall. "On the head-cavities and associated nerves in
Elasmobranchs." _Quart. J. of Micr. Science_, Vol. XXI. 1881.
(357) C. Schwalbe. "Das Ganglion oculomotorii." _Jenaische
Zeitschrift_, Vol. XIII. 1879.
_Sympathetic nervous system._
The discovery that the spinal and cranial nerves together with their
ganglia were formed from the epiblast was shortly afterwards extended
to the sympathetic nervous system, which has now been shewn to arise
in connection with the spinal and cranial nerves. The earliest
observations on this subject were those contained in my _Monograph on
Elasmobranch Fishes_ (p. 173), while Schenk and Birdsell (No. 361)
have since arrived at the same result for Aves and Mammalia.
[FIG. 274. LONGITUDINAL VERTICAL SECTION THROUGH PART OF THE BODY
WALL OF AN ELASMOBRANCH EMBRYO SHEWING PART OF TWO SPINAL NERVES AND
THE SYMPATHETIC GANGLIA BELONGING TO THEM.
_ar._ anterior root; _pr._ posterior root; _sy.g._ sympathetic
ganglion; _mp._ part of muscle-plate.]
In my account of the development of these ganglia, it is stated that
they were first met with as small masses situated at the ends of short
branches of the spinal nerves (fig. 275 _sy.g_). More recent
investigations have shewn me that the sympathetic ganglia are at first
simply swellings on the main branches of the spinal nerves some way
below the ganglia. Their situation may be understood from fig. 274,
_sy.g_, which belongs however to a somewhat later stage. Subsequently
the sympathetic ganglia become removed from the main stem of their
respective nerves, remaining however connected with those stems by a
short branch (fig. 275, _sy.g_). I have been unable to find a
longitudinal commissure connecting them in their early stages; and I
presume that they are at first independent, and become subsequently
united into a continuous cord on each side.
The observations of Schenk and Birdsell on the Mammalia seem to
indicate that the main parts of the sympathetic system arise in
continuity with the posterior spinal ganglia: they also shew that in
the neck and other parts the sympathetic cords arise as a continuous
ganglionic chain. The observations on the topographical features of
the development of the sympathetic system in higher types are however
as yet very imperfect.
The later history of the sympathetic ganglia is intimately bound up
with that of the so-called suprarenal bodies, which are dealt with in
another chapter.
[FIG. 275. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK
OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDER THAN FIG. 29 B.
The section is diagrammatic in the fact that the anterior
nerve-roots have been inserted for their whole length; whereas they
join the spinal cord halfway between two posterior roots.
_sp.c._ spinal cord; _sp.g._ ganglion of posterior root; _ar._
anterior root; _d.n._ dorsally directed nerve springing from
posterior root; _mp._ muscle plate; _mp´._ part of muscle plate
already converted into muscles; _mp.l._ part of muscle plate which
gives rise to the muscles of the limbs; _nl._ nervus lateralis;
_ao._ aorta; _ch._ notochord; _sy.g._ sympathetic ganglion; _ca.v._
cardinal vein; _sp.n._ spinal nerve; _sd._ segmental (archinephric)
duct; _st._ segmental tube; _du._ duodenum; _pan._ pancreas; _hp.d._
point of junction of hepatic duct with duodenum; _umc._ umbilical
canal.]
BIBLIOGRAPHY OF THE SYMPATHETIC NERVOUS SYSTEM.
(360) F. M. Balfour. _Monograph on the development of Elasmobranch
Fishes._ London, 1878, p. 173.
(361) S. L. Schenk and W. R. Birdsell. "Ueb. d. Lehre von d.
Entwicklung d. Ganglien d. Sympatheticus." _Mittheil. a. d.
embryologischen Instit. Wien._ Heft III. 1879.