In all the Sauropsida
the cerebro-spinal axis is angulated
at the junction of the spinal cord with the medulla oblongata,
the latter being bent down toward the ventral side of the
body. The region in which the nerves of the anterior and
posterior extremities originate may be enlarged in reptiles, as
in birds; but, in the former, the posterior columns of the cord
remain parallel in the lumbar enlargement while, in the latter,
they diverge and give rise to the sinus rhomboidals,
which is a sort of repetition of the fourth ventricle, the dilated
central canal of the spinal cord being covered merely by a
thin membrane consisting chiefly of the ependyma and arachnoid.
The brain (Fig. 90) fills the cavity of the skull in the
, and presents a well-developed cerebellum;
a mesencephalon divided above into two optic lobes; and
relatively large prosencephalic hemispheres, which attain a
considerable size in Crocodilia
, and Aves
, but never conceal
the optic lobes. In Crocodilia
the cerebellum presents a distinct
vermis, with transverse fissures. In birds the latter are
more distinct, and the lateral appendages of the cerebellum,
, become well defined, and are lodged, as in many
of the lower Mammalia
, in cavities of the side walls of the
skull, arched over by the anterior vertical semicircular
90. - A, C, the brain of a Lizard (Psammosaurus Bangalensis), and B, D, of a bird (Meleagris gallopuro, the Turkey), drawn as if they were of equal lengths. A, B,
viewed from above; 0, D, from the left side. Olf., Olfactory lobes; Pn., Pineal gland; Hmp., cerebral hemispheres; Mb, optic lobes of the mid-brain; Cb., cerebellum; M.O.,
medulla oblougata; ii. iv., vi., second, fourth, and sixth pairs of cerebral nerves; Py.,pituitary body.
There is no pons Varolii
, in the sense of transverse fibres
connecting the two halves of the cerebellum, visible upon the
ventral surface of the mesencephalon. The optic lobes contain ventricles.
, the optic lobes usually lie close
together upon the dorsal side of the mesencephalon, but in Aves
(Fig. 90 B, D) they are thrown down to the sides of the
base of the brain, and are connected over the aquaeductus
by a broad commissural band.
Each prosencephalic lobe contains a lateral ventricle (continuous
through the foramen of Munro with the third ventricle),
which is little more than a fissure between the very
thin inner wall of the lobe and its thick outer part, which contains
the corpus striatum. The corpora striata are united by
an anterior commissure, which is not of large size. The thinning
of the inner wall of the lobes, from the margin of the
foramen of Munro backward, which gives rise to the fissure
of Bichat in the Mammalia
, extends for a very short distance
in the Sauropsida
, even in birds.
The olfactory lobes are usually elongated, and contain ventricles
continuous with those of the prosencephalic hemispheres.
In all Sauropsida
the motor nerves of the tongue pass
through a foramen in the exoccipital bone. Hence, twelve
pairs of cranial nerves are present, except in the Ophidia
which possess no spinal accessory nerve.
|Fig. 91. - The brains of a Lizard (Psammosaurus Bengalensis) and of a bird (Meleagrins
gallopavo), in longitudinal and vertical section. The upper figure represents the lizard's
brain; the lower (taken, like Fig. 90, B, D, from Carus's "Erlauterunga-Tafeln")
that of the bird.
The letters ss in the preceding figure, except L. t, lamina terminalis, or anterior wall
of the tliird ventricle; f. M., foramen of Munro; a., anterior commissure; Th. E., thalamenephalon: s., soft commissure; p., posterior commissure; iv., indicates the exact point
of exit of the fourth pair from that part of the brain which answers to the value of Vieussens.
The lateral cutaneous branches so generally sent to the
trunk by the pneumogastric in the Ichthyopsida
but the pneumogastric gives a recurrent branch to the larynx.
The third, fourth, and sixth nerves arise quite independently
of the fifth.
The sympathetic is well developed, except in the Ophidia
where it is not distinct from the spinal nerves, in the greater
part of the trunk.
, many Sauria
, and Aves
, possess nasal glands,
which, in birds, attain a large size, and lie more usually upon
the frontal bone, or in the orbits, than in the nasal cavity.