Gilbert C. (Gilbert Charles) Bourne.

An introduction to the study of the comparative anatomy of animals (Volume 1) online

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roots, a dorsal or posterior, and a ventral or anterior root. The
dorsal root is also called sensory or afferent, because it is com-
posed of fibres, which only transmit impulses from without
inwards from the tissues to the spinal cord and brain. The
ventral roots are also known as motor or efferent, because their
fibres only transmit impulses in the reverse direction, from the
cerebro-spinal axis to the tissues. The two roots unite to
form one just as they pass out through the inter-vertebral
foramina, and the dorsal afferent root has an enlargement or
ganglion on it just before it joins the ventral root. The spinal
nerves are connected with the sympathetic nervous system,
but before describing this it will be well to consider the brain
and cranial nerves.

The brain is directly continuous with the spinal cord, and


a small canal, which traverses the centre of the latter, expands
into the brain to form a series of chambers known as the
ventricles of the brain.

The brain may he considered as consisting of three parts
the hind-brain, the mid-brain, and the fore-brain.

The hind- brain of the frog is a somewhat enlarged but
direct forward continuation of the spinal cord. It has two
divisions, the medulla oblongata (also called the bulb, or
myelencephalon) and the cerebellum (metencephalon). The
spinal cord, as it passes forward into the bulb, widens out,
its floor becomes thicker, its roof very much thinner, and its
central canal widens out to form a triangular cavity, the
fourth ventricle, whose exceedingly thin roof is covered over
by a very vascular membrane. The cerebellum, relatively
large in many vertebrates, is represented in the frog by a
narrow band of nervous tissue lying tranversely over the most
anterior part of the bulb. The rnid-brain (mesencephalon) lies
immediately in front of the hind-brain and in a line with it.
Its roof is formed by a pair of ovoid swellings, called the
optic lobes or corpora bigemina ; its floor, which is thick, con-
sists chiefly of nerve fibres running forward from the bulb to
the fore-brain. Each optic lobe is hollow, and its cavity com-
municates with a narrow passage called the Sylvian aqueduct,
leading from the cavity of the fourth ventricle to the cavity of
the fore-brain in front. This passage is sometimes called the
" iter." Iter a tertio ad quartum ventriculum.

The fore-brain consists of two parts, the thcilamencephalon
behind, and the cerebral hemispheres or prosencephalon in
front. The side walls of the thalamencephalon are thickened
to form the optic thalami, but its roof and floor are thin, and
the former is covered over by a very vascular membrane,
just as is the roof of the fourth ventricle. By reason of the
thickening of its lateral walls, and the thinning out of its floor
and roof, the cavity of the thalamencephalon, known as the
third ventricle, is deep dorso-ventrally but narrow from side
to side. Its floor is produced into a conical depression, the
infundibulum, and its roof is produced into a hollow finger-
like projection, on the top of which is borne a rounded
vascular body usually known as the pineal gland. It appears,
however, that what is usually called the pineal gland is nothing
more than a thickened portion of the choroid plexus or


vascular membrane which covers this part of the head. The
true pineal gland is a small vesicle lying outside the skull
beneath the skin. In the embryo this vesicle is connected by
a pedicle with the roof of the thalamencephalon, but in the
adult frog the connection is lost, and there is no foramen
between the parietal bones whereby the pineal gland can make
communication with the brain. All that remains inside the
skull, therefore, is the hollow pedicle. The pineal body is
a structure of much interest which occurs in the brains of all
Craniata. The older anatomists fancifully described it as
being the seat of the soul, but it really is the relic of a once
well-developed and functional sense-organ, having the character
of an eye, with lens, retina and nerve, the last being re-
presented by the pedicle. Such an eye, in a more or less
degenerate condition, actually occurs in several living reptiles,
and in them there is a distinct foramen between the parietal
bones through which the nerve or pedicle passes to the eye.
This parietal foramen is obsolete in living Batrachia, but was
universally present in certain extinct Amphibia the Stego-
cephalia, and it is inferred that in them the pineal eye was
well developed and functional. It is worthy of remark that
the pineal organ, though it has the structure of an eye, does
not resemble the paired eyes of Vertebrata, but rather re-
sembles those of certain Invertebrata.

In front the third ventricle is bounded by a wall of nervous
tissue called the lamina terminalis. Right and left of this a
passage leads into the cavities of the foremost division of the
fore-brain, the cerebral hemispheres, which together constitute
the prosencephalon. The hemispheres are ovoid bodies of
considerable size relatively to the other parts of the brain.
Their smaller ends are directed forward, and produced in front
into two rounded swellings, the olfactory lobes, or rhinen-
cephala. The hemispheres are united together before and
behind by the fusion of their inner walls, but are completely
separated from one another in their middle portions by a deep
vertical cleft extending from the dorsal to the ventral surface.
The rhinencephala are fused together by their inner walls, but
both in them and in the fused portions of the cerebral hemi-
spheres, shallow median furrows on the dorsal and ventral
surfaces mark their double origin.

The cavities of the cerebral hemispheres are known as the


lateral ventricles. They are separate from one another, but

Fig. ii.

The Brain, Cranial Nerves, and anterior part of the Spinal Cord
of the Frog dissected from the dorsal surface. Cb, cerebral
hemispheres ; Thai, Thalamencephalon ; in the middle of
it is seen the hole by which the stalk of the pineal body
communicated with the cavity of the third ventricle ; O L,
corpora bigemina (mid-brain) ; Cer, cerebellum ; B, the
bulb or medulla oblongata ; 4]', the fourth ventricle ; /,
the olfactory or first pair of cranial nerves ; //, the optic,
second pair of cranial nerves ; Va, the ophthalmic branch of
the trigeminal or fifth pair of cranial nerves ; Vb, the
maxillary branch, and Vc, the mandibular branch of the
trigeminal. The two last branches spring from a large
swelling on the main stem of 'the nerve ; this is the
Gasserian ganglion ; VI I ^ the main branch of the facial
or seventh pair of cranial nerves crossing over Col, the
columellaauris, and just beyond this sending back a branch
to connect with the ninth nerve ; rp, the ramus palatinus
of the seventh nerve, passing below the eyeball and
distributed to the skin on the snout and anterior part
of the palate ; VIII, the auditory or eighth pair of cranial
nerves; /A', the glossopharyngeal or ninth pair of cranial
nerves ; X, the pneumogastric, vagus, or tenth pair of
cranial nerves. The vagus has a large ganglion on its
root which is joined by the anterior extension of the
sympathetic system coming from the first sympathetic
ganglion lying under Hy y the hypoglossal or first pair of
spinal nerves ; Br, the brachial second pair of spinal
nerves. The third, fourth, and sixth pair of cranial nerves
passing to the muscles of the eyeball are not shown in the

each opens by a short narrow passage into the third ventricle


just behind the lamina terminalis. The Y-shaped opening
thus produced is known as the foramen of Munro. The
lateral ventricles are ovoid cavities with rather thin walls in the
embryo, but, as growth proceeds, the anterior and inner part
of the wall of each becomes much thickened, and bulges into
the cavity, reducing it to a crescentic slit in its lower portion.
The thickened prominences are known as the corpora striata.

Ten pairs of cranial nerves are given off from the brain.

The first pair springs from the anterior end of the
rhinencephala, and passes straight to the olfactory chambers ;
these are the olfactory nerves. The second or optic pair
arises from the sides of the brain beneath the optic lobes.
Each nerve starts as a broad band of fibres which runs forward
and downward to meet its fellow of the opposite side on the
under surface of the thalamencephalon just in front of the
infundibulum. Here most if not all of the fibres of the optic
nerves cross over to the opposite side, the point of their
decussation being called the optic chiasma. From the chiasma
each nerve runs outwards through a foramen in the cranial
wall, and passes into the eyeball.

The third; fourth, and sixth pairs of cranial nerves are dis-
tributed to the muscles of the eye.

The eyeball is moved by six muscles passing from its equator
to the walls of the orbit. Four of these, attached close
together to the inner posterior angle of the orbit, are known
as the recti muscles, and are attached respectively to the
upper (rectus superior), lower (rectus inferior), posterior
(rectus posterior), and anterior (rectus anterior) sides of the
eyeball. In addition to these a muscle arising from the
anterior inner angle of the orbit passes obliquely backward,
and is inserted on the lower surface of the eyeball (inferior
oblique), and another (the superior oblique), arising close to
the origin of the inferior oblique, passes obliquely upward and
backward and is inserted on the upper surface of the eyeball.
These muscles occur in all craniate vertebrates ; the frog has in
addition a musculus retractor bulbi, which partly surrounds
the optic nerve, and lies within the cone formed by the four
recti muscles. The third nerve, called the motor oculi,
supplies the recti superior, inferior, and anterior, and the
obliquus inferior. The fourth nerve supplies the superior
oblique, and is known as the pathetic or trochlear nerve. The


sixth, or abducens nerve, passes to the posterior rectus and gives
off a branch to the retractor bulbi. The third nerve rises from
the floor of the mid-brain near the median line, the sixth pair
from the ventral surface of the medulla, and also close to the
median line ; but the fourth pair differs from all the other cranial
nerves in arising from the dorsal side of the brain between the
medulla and optic lobes.

The fifth pair of nerves, called the trigeminal, is the largest
of the cranial nerves of the frog. The nerve of each side
arises from the side of the anterior end of the medulla
oblongata, runs outward and forward to the cranial wall, and
passes through a foramen which is partly bounded by a notch
in the inner side of the pro-otic bone. Just before it reaches
the foramen the trigeminal swells out to form the large
Gasserian ganglion. After passing through the foramen the
nerve runs along the anterior face of the auditory capsule and
divides at once into two large branches, the ophthalmic branch
and the maxillo-mandibular branch. The ophthalmic branch
runs forward close to the cranial wall between it and the
eyeball. At the front end of the orbit it divides into two
nerves which pass through apertures in the walls of the olfactory
capsule and supply the nose and the skin of the front of the
head. The maxillo-mandibular nerve, after a short course in
front of the auditory capsule, divides into two branches an
upper, the maxillary nerve, which runs forward and outward
between the eyeball and the lower and outer wall of the
orbit to the margin of the upper jaw, and the lower mandi-
bular nerve, which turns backward, outward, and downward
to the squamosal bone, and passes across to the lower
jaw, where it runs forward on the outer side to the

The seventh, or facial nerve, arises from the medulla, close
behind the fifth, and runs forward close to the Gasserian
ganglion, with which it unites. It leaves the cranium in close
company with the mandibular branch of the trigeminal, and
divides at once into (i) a palatine branch, which runs forward
on the floor of the inner side of the orbit, and eventually
makes connections with the maxillary branch of the fifth, and
supplies the roof of the mouth ; (2) a hyomandibular branch,
which runs outward and backward across the wall of the
auditory capsule to reach the columella auris ; crossing over


the inner end of this, it turns down to the eustachian tube and
angle of the mouth. The seventh is joined close to the
columella by a branch from the ninth.

The eighth, or auditory nerve, arises from the medulla,
immediately behind the seventh, goes straight through a
foramen in the auditory capsule to the ear, and supplies the
organ of hearing.

The ninth, or glossopharyngeal nerve, arises, in common
with the tenth nerve, from a number of roots on the side of the
medulla, behind the auditory nerve. It passes, in company
with the tenth nerve, through a foramen at the back of the
cranium just in front of the occipital condyle, and divides into
two branches, an anterior, which runs forward to join the facial
nerve close to the columella, and a posterior, which curves
downward and inward to the floor of the pharynx, along
which it runs, supplying the petrohyoid muscles and the
mucous membrane of the tongue and pharynx.

The tenth, pneumogastric or vagus nerve, is a very
important nerve with complicated relations. After leaving the
skull in company with the ninth nerve, it enlarges to form a
ganglion, the ganglion nervi vagi. The nerve runs backward
and ventralward along the side wall of the pharynx, and
finally divides into several branches, of which the most
important are : (i) the nervus recurrens or laryngeus, which
arrives at the posterior cornu of the hyoid, and turns forward to
pass beneath it and the pulmo-cutaneous artery, whence it runs
forward close to the middle line, to end in the larynx; (2)
the ramus cardiacus, which runs along the dorsal surface of
the pulmonary artery and the superior vena cava towards the
sinus venosus, where it joins its fellow of the opposite side, and
the two pass on to the auricular septum of the heart ; (3) the
rami gastrici, which run through thepartial diaphragm formed by
the insertion of the internal oblique muscle upon the walls of the
oesophagus, and are distributed to the walls of the stomach ; (4)
the rami pulmonales, which also perforate the partial diaphragm,
and then follow the course of the pulmonary artery to the lung.

The sympathetic nervous system (see fig. 10) is intimately
connected with both spinal and cranial nerves. It is a chain of
paired nervous ganglia, joined together by longitudinal cords,
lying on either side of the vertebral column, and each ganglion
makes connection with a spinal nerve by a short branch, the


ramus communicans. In the anterior part of the body the
sympathetic chains lie right and left of and parallel to the
vertebral column ; at about the level of the sixth vertebra they
approach one another in the middle line, and become closely
connected with the dorsal aorta, alongside of which they run.
The first sympathetic ganglion lies on the hypoglossal nerve,
close to its exit from the first inter-vertebral foramen. Its
ramus communicans is represented by a few fine and very
short fibres connecting the ganglion with the nerve. From
the ganglion two or three nervous strands pass backward, one
passing beneath and one or two above the brachial artery, so
as to encircle it, as it were, with a ring. This ring is known
as the annulus of Vieussens. The second ganglion lies on
the brachial nerve ; it is the largest of the whole series, and,
like the first, is connected with the spinal nerve by a few fine
fibres which do not form a distinct ramus communicans.
The third ganglion is usually fused with the second, but has
a short and distinct ramus communicans with the third spinal
nerve. After this point the sympathetic chain follows the
course of the systemic aortic arch, and when this joins its
fellow to form the dorsal aorta the sympathetic chains follows
the course of the latter. Being thus farther removed from the
vertebral column, the rami communicantes become longer.
There are as many sympathetic ganglia as there are spinal
nerves, and each makes communication by its ramus com-
municans with the spinal nerve proper to it. Thus the fourth,
fifth, sixth, and successive spinal nerves up to the tenth,
supply each a ramus communicans to a sympathetic ganglion,
the tenth nerve being peculiar in making several connections
as many as ten or twelve in some cases with the sympathetic
chain, but the number is not constant. The seventh, eighth,
and ninth are also said to have two or three communications
apiece with the sympathetic chain.

The sympathetic chains of each side are connected by
numerous fine twigs, which surround the dorsal aorta and
form a plexus around it, and from this plexus very fine nerves
pass to the adjoining organs. The sympathetic system is
characterised by the fact that its branches divide and sub-
divide, and the sub-divisions interlace and anastomose with
one another to form networks or plexus which include
numerous ganglia. The two most important of these are:


(1) the cardiac plexus, formed by nerves arising from the first
ganglion of the chain. It lies on the auricles and surrounds
the great blood-vessels at their openings into the heart;

(2) the solar plexus, formed by branches from the third, fourth,
and fifth ganglia, and lying on the dorsal side of the stomach.
Anteriorly the sympathetic chain makes communications with
the cranial nerves by a branch which passes on either side
from the first ganglion to the ganglion nervi vagi, where some
of its fibres enter the vagus, the remainder pursuing their
forward course to enter the Gasserian ganglion of the fifth

We may nere consider two collections of nervous ganglia
in the heart which are connected with the vagi and the
sympathetic system. The two vagi pass to the sinus venosus
and enter a nervous ganglion situated in its wall and known
as Remak's ganglion. Thence the vagi pass into the auricular
septum, one in its dorsal and the other in its ventral portion,
and run backward to the region of the auriculo-ventricular
groove where they enter another collection of nerve-cells
known as Bidder's ganglia. From these ganglia fibres are
distributed to the rest of the heart. These two minute collec-
tions of nervous matter are of importance to physiologists.

The eyeball of the frog is attached to the skull by muscles
which have already been described. The eyeball is not
spherical, but is flattened on its outer side, more convex on its
inner side. It consists of the following parts : (i) a firm outer
wall, the sclerotic, formed of cartilage and dense white con-
nective tissue ; (2) the cornea, a transparent area on the ex-
posed part of the eyeball, through which light is admitted into
its interior: it is continuous with the sclerotic ; (3) a coloured
curtain, the iris, lying behind the cornea, and surrounding a
central aperture, the pupil. The iris is provided with circular
and radial muscle fibres, by means of which tHe size of the
pupil can be diminished or enlarged. In the interior of the
eye a firm transparent spheroidal body lies behind the iris,
attached to its outer margin. This is the lens, and it serves to
focus light upon the back of the eye. Between the cornea and
the lens is a small space, the anterior chamber of the eye, filled
with a watery fluid, the aqueous humour. Between the lens
and the back of the eye is a much larger space, the posterior
chamber of the eye, filled with a gelatinoid material, the



vitreous humour. The sclerotic is lined by a black pig-
mented layer called the choroid, which is continued in front
into the iris. Inside of this, lining the chamber of the eye, is
a delicate transparent membrane, the retina, on which the
ramifications of the optic nerve are spread. It is the sensitive
portion of the eye, and the cornea, iris, lens, etc., are structures
adapted to focus the light upon it.

The frog's ear is embedded in the periotic capsule, which, as
we have seen, is largely cartilaginous, with an anterior ossifica-
tion, the pro-otic. The essential organs of hearing are deeply


Fig. 12.

The right membranous labyrinth of the frog's ear
seen from the inner side. 17, utriculus, from which
spring ace, the anterior semicircular canal with
its ampulla aa : pec, the posterior circular canal
with its ampulla amp ',, the external
horizontal semicircular canal, with its ampulla
amp, represented as showing through the trans-
parent walls of the utriculus ; S, the sacculus ; de,
the ductus endolymphaticus ; lag, lagena ; pbc,
pars basilaris cochleae. (Adapted from Retzius.)

embedded in the cartilage, and only communicate with the
surface by an accessory apparatus consisting of the tympanic
cavity, the eustachian tube, and columella.

The tympanic cavity, bounded externally by the tympanic
membrane, is seen, after removal of the latter, as a shallow,
funnel-shaped cavity, lined by a pigmented mucous membrane,
and communicating by the wide, short passage of the eusta-
chian tube with the pharynx. The inner and deeper part
of the cavity presents a small aperture leading into the inner
ear. This aperture, called the fenestra ovalis, is blocked
by the enlarged cartilaginous end of a partly bony, partly
cartilaginous rod, the columella auris, which traverses the


tympanic cavity and is fixed by its outer end in the tympanic
membrane. The tympanic cavity and eustachian tube
together constitute the middle ear, and are derived from the
most anterior of the gill-clefts of the tadpole. The inner
ear consists of a membranous sac lying in a corresponding
cavity of the cartilage of the auditory capsule. Between the
cartilage and the sac is a fluid called perilymph. The sac
also has fluid contents called endolymph. A constriction
partly divides the sac into two portions: (i) an upper and
larger division, called the utriculus; (2) a lower and smaller divi-
sion, with three small dilatations on its posterior face, called
the sacculus. The last named also gives off from its inner
and upper border a tubular offset ending in a thin-walled
dilatation. This is called the ductus endolymphaticus.

The utriculus has more complex relations. Three semi-
circular canals, an anterior, a posterior, and an external, open
into it. Of these the anterior semi-circular canal lies in the
median or sagittal plane of the head ; at its anterior end, just
where it joins the utriculus, it is dilated to form an ampulla,
and its posterior end joins the posterior semi-circular canal
which opens, in common with it, into the utriculus. The
posterior semi-circular canal lies in the transverse plane ; its
upper end opens into the utriculus, in common with the
anterior canal, and its lower end dilates into an ampulla be-
fore opening into the utriculus. The external canal lies in the
horizontal plane, and has an ampulla at its anterior end. Thus
the three canals lie at right angles to one another in the three
dimensions of space ; they open into the utriculus at both
ends, and each has an ampulla at one end. The auditory
nerve, passing through an aperture in the inner wall of the
auditory capsule, divides into branches, which are distributed to
the utricle, the saccule, and the ampullae. In the interior of
the membranous labyrinth are the elements sensitive to sound-
waves, in the form of modifications of the lining epithelium
bearing stiff sensory hairs. There are also peculiar calcareous
concretions, called otoliths, especially abundant in the ductus

The olfactory organs of the frog consist of a pair of
sacs separated from one another by a median cartilaginous
septum, and opening to the exterior by the anterior nares, into
the buccal cavity by the posterior nares. The cavity of each


sac is divided up and complicated by the projection into it of
cartilaginous offsets of the walls of the nasal chambers, and
three main subdivisions or sinuses have been recognised in
each sac a dorsal, a ventral, and a lateral sinus. These
sinuses are lined by an epithelial membrane which in the
olfactory region is modified to serve as the percipient

We have thus far treated of the anatomy of the frog as it

Online LibraryGilbert C. (Gilbert Charles) BourneAn introduction to the study of the comparative anatomy of animals (Volume 1) → online text (page 7 of 27)