the fifth digit. The claws of many geckos are “retractile,” like those of cats; the adhesive lamellae on the under side of their digits have already been described (see Gecko).
Nervous System.
The hemispheres are still much longer than broad, and pass, especially in lizards, gradually into the olfactory lobes, into which continue the ventricles of the hemispheres. The dorsal walls of these are thin, especially in crocodiles, although they possess already a considerable amount of grey matter. The basal masses of the fore-brain bulge into the roomy ventricles like cushions. Fibres referable to a corpus callosum are scarcely separated from those of the still much stronger anterior commissure. The epiphysis comes to the surface between the hinder parts of the hemispheres. The pineal eye is described below under Sense Organs. The hypophysis has but a shallow infundibulum. The mid-brain shows a pair of dorsal globular swellings, each with a cavity; they separate the hemispheres from the cerebellum. Of the hind brain, the middle portion is by far the largest; although the dorsal wall of this cerebellum is thick, and rich in grey matter, its surface is still quite smooth and it shows no trace of an arbor vitae. It covers but a small portion of the wide fourth ventricle.
The spinal cord shows a brachial and a lumbar longitudinal swelling, especially marked in tortoises, but without a rhomboidal sinus. The cord is continued into the end of the tail.
The cranial nerves of the reptiles agree in their arrangement
and distribution more with those of birds and mammals than
with those of the Batrachia. The facial nerve sends a palatine
branch to the palate and to the superior maxillary of the
trigeminus, and a strong mandibular branch joins the third of the
trigeminal, and further ramifications supply the sphincter
muscle of the neck. The vagus and glossopharyngeus leave
the cranium separately. The vagus then goes towards the
Fig. 36.—Brain of Lacerta agilis. (After
Leydig.) 1, Dorsal aspect; 2, vertical
longitudinal section. cb, cerebellum; ch,
cerebral hemisphere; m, medulla oblongata;
olf, olfactory lobes; on, optic nerve;
opl, optic lobes; p, pineal body or
epiphysis; py, base of pituitary body.
heart, which in the
Sauropsida is far
removed from the head,
and there possesses
another ganglion, variously
called ganglion
trunci vagi or g.
nodosum. It is
connected by a nerve
with the large ganglion
supremum of the
sympathetic. From
the cardiac ganglion,
and from the
continuation of the
vagus, are sent off
several branches in
succession, which,
having to pass below
or tailwards from the transverse carotic, aortic and Botallian
vessels, have to take again a headward course to the
larynx and pharynx; a side branch enters the heart by its
truncus. The main mass of the vagus then supplies
lungs, stomach and further viscera. The accessory or 11th
cranial nerve arises with about half a dozen roots which extend
often beyond the second cranial nerve; they collect into a
thin stem which leaves the cranium together with the vagus,
with which it is often fused; it supplies the cucullaris s. trapezius
muscle.
The hypoglossus arises by two ventral roots, leaving the skull by two holes through the lateral occipital bone, near the condyle. The united stem is invariably joined by strong branches from cervical nerves, always from the first, mostly also from the second, sometimes also from the third. The details vary much; occasionally there are three cranial roots and foramina, and then only the first cervical joins the hypoglossus; this often fuses with the glossopharyngeal or with the vagus. In the broad and well-muscularized tongue of the crocodiles the right and left hypoglossal branches form a complete ansa, an arrangement in which A. Schneider saw the infraoesophageal nerve ring of Invertebrata!
The spinal nerves each issue behind, or through, the neural arch of the vertebra to which they belong genetically. The first spinal, or suboccipital, nerve has no dorsal roots, and, having lost its vertebra, an apparently anomalous arrangement has come to pass, in this way, that there are x cervical vertebrae, but x + 1 cervical nerves, a condition prevailing in, and characteristic of, all Amniota. The hypoglossal-cervical plexus is separated from the brachial plexus by several meta meres, according to the length of the neck. The brachial plexus is composed of about 5 nerves; the variations have been studied chiefly by M. Fürbringer. It is interesting to note that the brachial plexus still persists in snakes, although they have completely lost the anterior girdle and the limbs (Albertina Carlsson). A disturbance in the pelvic region likewise indicates in snakes the former existence of a pelvic or lumbo-sacral plexus, which in limbed reptiles is composed of about 5 nerves, the last of which is weak and in many cases (by no means the rule) issues between the two sacral vertebrae, sending one branch to the ischiadic, another to the public plexus which supplies the cloacal region. (For details of these plexuses see the papers by Mivart, Jhering and Gadow.)
The sympathetic system shows considerable modifications in the various orders and even families of the reptiles. In the neck region, in Sphenodon and most lizards it is, on the right and left side, composed of two portions. One, more lateral and placed deeply, runs along the side of the vertebral column, starting from the first and second spinal nerves, with which it is connected by so-called rami communicantes; it is not connected with the other spinal nerves until it reaches, in the thorax, the first stem of the brachial plexus, and hereabout lies the so-called second thoracic ganglion. The other, superficial and more ventral, portion arises from the petrosal ganglion of the glossopharyngeal, and from the vagus ganglion, and then forms a long loop which joins the second thoracic ganglion. In its long course it sometimes, e.g. in Varanus, forms one common stem with the vagus before it splits off. At a variable distance, but not far above the heart, the vagus possesses a big swelling, the ganglion trunci vagi, and the sympathetic stem, in the same level, or farther down, has likewise a large ganglion, the g. supremum vagi, or first thoracic ganglion. The vagus ganglion receives several nerve strands from this big sympathetic ganglion, and then divides as described above.
In the crocodiles the deep portion of the sympathetic begins at the vagus and extends in rope-ladder fashion into the thorax, there being, as in birds, regular transverse communicating branches with the spinal nerves, and the longitudinal strands run through the transverse foramina between the capitular and tubercular portions of the cervical ribs. The other, ventral, portion starts by a right and a left branch from the vagus ganglia, but both branches unite at once into one unpaired stem, which is deeply embedded in the middle line between the ventral muscles of the cervical vertebrae. Very thin branches connect this unpaired stem with the right and left sympathetic portions; small ganglia are embedded in the unpaired nerve.
The so-called second thoracic ganglion is in reality a compound of all the sympathetic ganglia of the four or five metameres of the brachial plexus. It forms the point of juncture of the deep and the superficial cervical sympathetic portions. From the posterior region of the thorax backwards the right and left strands run along their side of the vertebral column, with a communicating branch and a ganglion for each metamere; sometimes one or more successive ganglia are combined, for instance near the cloaca. After having supplied the latter, the sympathetic system appears exhausted and is continued into the tail by but a very thin strand, which runs between the caudal vein and artery. The best illustrations of the sympathetic system are those by Vogt (neck of crocodile), J. G. Fischer (many
