856 
ZOOLOGY. 
rays and sharks. The bulk of the whole brain is very in¬ 
considerable. As to the form of the individual portions of 
the brain, we find the first cerebral mass here, as in the 
following classes, constantly provided with the cavity which 
was first developed in the shark. In the salamander and 
frog it is tolerably elongated ; and in the latter the two gan¬ 
glia are, as well as their cavities, united into one, whilst, pos¬ 
teriorly, they are connected by a medullary band. Within 
eaph of the cavities corresponding to the lateral ventricles of 
the human brain, there is here, as well as in all other amphi¬ 
bia, a swelling very similar to the corpus striatum of man. 
In the second cerebral mass we find the optic tubercles 
smaller, and with a more simple cavity; partly forming a 
repetition of the type of rays and sharks, and partly approxi¬ 
mating to the form of the anterior pair of the corpora qua- 
drigemina in man. Besides the proper optic tubercles, how¬ 
ever, there isp in all the orders of this class, a pair of smaller 
ganglia placed before them, which correspond to the optic 
thalami, as they are usually called, or ganglia of the hemi¬ 
spheres, in the brain of man, and, like them, give some fibres 
to the optic nerves. The true optic tubercles are usually 
consolidated into one mass; and, in the frog, as in osseous 
fishes, contain a swelling, from which the medullary fibres 
radiat§ to the covering of the tubercles. In tortoises, lizards, 
and serpents, the cavity of the optic tubercles is smooth. A 
small pineal gland is always found on the ganglia of the 
hemispheres. 
The third cerebral mass consisting of the cerebellum 
and medulla oblongata, is very simple in frogs and serpents. 
The cerebellum forms only a narrow medullary band, co¬ 
vering the fourth ventricle ; to the back part of it a vascular 
lamina is attached, forming, as in the lamprey, the choroid 
process of the fourth ventricle, which, in serpents, is very 
small. The nerves are distributed in the same manner as in 
man ; those only, as must evidently be the case, being absent, 
which correspond to organs not yet in existence; e. g. 
phrenic nerves, because there is no diaphragm; nerves of the 
pelvis and extremities in serpents, because those parts are 
wanting. Of the cerebral nerves, the maxillary and vagus 
are still distinctly recognisable as the intervertebral nerves 
of the head. The auditory is a separate cerebral nerve, in 
this class of animals, and of considerable size in tortoises 
and lizards. The optic nerves also are large, when the eye 
is pretty much developed, as in tortoises, the iguana, &c. 
In the latter, a transverse incision at the commissure of the 
optic nerves, clearly proves their decussation by the mutual 
interlacement of medullary laminae. The olfactory nerves 
are but short in serpents, terminate in a club-like swelling, 
and, in most amphibia, appear as uninterrupted prolongations 
of the first cerebral lobes. 
The sympathetic nerve has nearly the same course in am¬ 
phibia as in fishes. 
52. The brain of the coluber haje; Nos. 1,2, medulla spi¬ 
nalis; 3, the cerebellum; 5, optic lobes; 6, the pineal 
gland, and posterior part of the hemispheres; 7, anterior 
part of the hemispheres; 8, olfacloiy pedicle; 9, olfactory 
lobule. 
In birds the brain and spinal marrow are developed with 
singular uniformity in different species, and comparatively 
with the body in general, in a higher degree than in any in¬ 
stance we have yet mentioned. The brain is more decidedly 
distinguished from the spinal marrow by its greater breadth 
and more globular form. 
The spinal marrow extends into the caudal vertebra;. It 
is, however, considerably reduced in size, at the same time 
that the column of caudal vertebra; is comparatively short. 
In fact, the inferior swelling on the spinal marrow, is con¬ 
tained within the sacral vertebrae, and its continuation is 
little more than a terminal fibril, extending into the tail, 
and giving off some few pairs of nerves through the holes 
between the caudal vertebra;. One of the principal points 
to be noticed in the brain of birds is, that the three pri¬ 
mary divisions belonging to it are no longer placed one 
behind the other, as in the two preceding classes, but are si¬ 
tuated one below the other. The anterior mass appears already 
to occupy a higher rank than that of mere olfactory ganglia. 
It still, however, consists chiefly of gray substance, and, as 
in the preceding classes, is principally connected with its 
fellow by a narrow commissure; besides this, however, 
there is a small soft commissure placed above it, which 
Meckel, who first discovered it, considers as a rudiment of 
the great commissure of the brain in the following class, and 
which Carus looks on as more particularly corresponding to 
the anterior inflected portion of the corpus callosum, a part 
which is only found very strongly marked in the rodentia 
among mammalia. Within the ventricles are large swellings, 
corresponding to the corpora striata of man. The form of 
the hemispheres varies to some extent in the different species 
of birds. In passerine birds they are usually long and 
broad, completely covering the optic tubercles; in accipitres, 
on the contrary, the optic tubercles project considerably be¬ 
side and behind them, although, at the same time, the hemi¬ 
spheres are of remarkable breadth ; in some aquatic birds, the 
duck for instance, they are, on the contrary, rather oblong. 
The olfactory nerves always arise from their anterior part, 
with the intermedium of a swelling on each side, a medullary 
stria running along the lower surlace of each hemisphere to 
the origin of the corresponding nerve. 
In order completely to delect the coincidence of form be¬ 
tween the second mass or division of the brain of birds and 
the corresponding part in amphibia, it is only necessary to 
consider the brain of a nearly perfect embryo of a bird as 
intermediate between them. Here (in the embryo) the he¬ 
mispheres, as in the amphibia, are, comparatively, smaller 
and narrower, whilst the optic tubercles are found lying close 
together, and immediately behind them, which is not the 
case in the full grown bird. In the latter, where, as in am¬ 
phibia, we may distinguish between the ganglia and the 
proper optic tubercles; these last are situated more laterally 
and inferiorly, being pressed aside by the greater expansion 
of the hemispheres, but in such a manner, that they are still 
connected by a medullary membrane, corresponding to the 
roof of the aqueduct in the human brain. They are smaller 
in proportion to the rest of the brain ; in which respect, as 
well as in the abundance of medullary substance covering 
their internal and external surfaces, they approximate to the 
anterior pair of the corpora quadrigemina in man. Their 
cavities are small, and open into the space beneath the me¬ 
dullary commissure of the optic tubercles above mentioned. 
As to the ganglia of the hemispheres, to which different 
writers have assigned very various characters, if we carefully 
consider the series of formations of the brain in the various 
stages of its progressive advance towards perfection, it will 
not admit of a doubt that they correspond absolutely to 
what have been called the optic thalami in the human 
brain. 
The cerebellum, which is the principal portion of the 
third cerebral mass, here, as we found the case in certain 
cartilaginous fishes and amphibia, consists of lamina dis¬ 
posed in transverse folds, and covering the fourth ventricle; 
with this difference, however, that its structure is much more 
perfected. The cerebellum in birds is very similar to the 
vermiform or central portion of the same part in man. Here, 
as in some fishes and amphibia, are lateral appendages, 
which, however, must not be identified with the great lateral 
lobes of the human cerebellum, but rather with the parts 
which Riel has called “ flocken.” The ganglia of the audi¬ 
tory nerves are also very distinct below the cerebellum. 
In the spinal and cerebral nerves we find but few striking 
peculiarities, the nerves being distributed to the different 
parts of the body, according to the same general plan as in 
man. It has already been mentioned that the olfactory 
nerves arise from the anterior extremities of the hemispheres. 
The optic nerves are generally very bulky, admitting of 
comparison in this respect only with those of some lizards. 
They arise from the whole of the external surface of the optic 
tubercles, and form a perfect decussation in the region of the 
infundibulum. 
The sympathetic nerve is placed at each side of the whole 
vertebral column, and has a ganglion at each vertebra, pecu¬ 
liarly 
