Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

104 ANATOMY.
Compara- these circumstances fully, we shall confine our attention is marked by transverse parallel strise corresponding to Compara-
^ve to the notice of a few only. the lamina of the mammiferous brain. There is neither tive
Anatomy, jj. js not eaSy to ascertain the proportion of the mass of middle band {corpus callosum), vault, nor septum. The
V ^ v' the brain to that of the rest of the body. Excluding as ceiling or vault of the aqueduct or passage from the third W'Y'W
much as possible the ordinary sources of fallacy, in small to the fourth ventricle is not, as in the Mammalia, sur-
animals the brain is proportionally larger; yet in this mounted by the bigeminous eminences, but is merely a
respect man is surpassed only by a small number of ani- thin plate corresponding to the valve. Each optic emi-
mals, habitually lean, and with little muscle, as bats, small nence contains a cavity communicating with the others
birds, &c. While the proportion of brain in man to that by the Sylvian aqueduct. The anterior eminences (cor-
of the whole person varies from a 22d to a 35th part, pora striata) are not striated with alternate white and gray
that of the monkey tribe varies from a 22d to a 42d part; matter, as in the Mammalia. Between the anterior and
and in the baboon it is only the 104th part of the body, the optic eminences are four rounded tubercles, best seen
Among the Mammalia, the Rodentia have in general in the ostrich, which are to be regarded as entirely hete-
the largest proportion of brain, and the Pachydermata rologous to the structure of the mammiferous brain, and
the smallest; and while the hare has a brain about the connecting the cerebral structure of Birds with that of
300th part of the size of the body, that of the elephant, Reptiles and Fishes, in which also these tubercles are ob-
the most sagacious of animals, is about the 500th part the served.
size of his body. It is also remarkable, that while the brain The Reptile brain is smooth and unconvoluted. The
of the horse is only a 400th part of the size of his body, optic eminences, which are situate behind the hemispheres,
that of the ass amounts to a 254th part. The Reptile brain are uncovered, and contain a ventricle communicating
becomes excessively small, that of the turtle being rather with the third. At the extremities of the latter are the
more than the 5000th part t>he size of his body; and anterior and posterior commissures, but there is neither
in some of the Fishes, not all, it appears to attain the soft commissure nor bigeminous eminences. The hemi-
maximum of decreasing proportion, that of the tunny be- sphere presents an anterior eminence, which, however, in
ing so small as the 37,000th part of his body, while the the brain of Birds is unstriated. The cerebral valve is,
brain of the carp is so large as to approach the proportion like that of Birds, unsurmounted by bigeminous emi-
of the elephant. It may be doubted whether, under such nences.
circumstances, any precise conclusions can be drawn from In the class of Fishes the structure becomes still more
results so variable and so little to be expected. simple. The tubercles of which the brain consists are
The proportional weight of the brain to that of the cere- placed in a row ; and their increase in number only de-
bellum is, excepting in the case of one species of ape, the monstrates the decomposition of the organ, and its reso-
saimiri, greater in man than in any other animal. The lution into simple integrant parts. The two representing
ox is equal to man in this respect, and the dog approaches the hemispheres are ovoidal, unconvoluted, and contain a
him. The animals most remote are the Rodentia, as ventricle, in which is seen the eminence analogous to the
the beaver, rat, and mouse, &c. striated bodies. The optic eminences, situate beneath
The convolutions, which are so numerous and so deep the hemispheres, though small, contain each a cavity, as
in man, diminish both in number and size in the Quad- in the two oviparous classes already noticed. Lastly,
rum an a and Carnivora, and are nearly obliterated in there are in several genera, under the common vault of
the Rodentia. In the Ungulated animals, however, the hemispheres, occasionally two, occasionally four tu-
and especially in the Ruminants and the horse, the con- bercles, variable in shape and proportions, but which
volutions are numerous; and even in the dolphin among would be analogous to the bigeminous eminences, were
the Cetacea, they are numerous and deep. In all the they not, like those already mentioned in Birds, situate
Mammalia the cerebellum is foliated. before and above the optic chambers. In the cartilagi-
On the whole, the peculiar character of the brain of nous fishes, in which these tubercles are not observed,
man and the ape family consists in the existence of the the anterior or striated eminences are obliterated. The
posterior lobe and digital cavity. The brain of the Zoo- cerebellum does not cover the fourth ventricle.
phaga is remarkable for the small size of the nates or Behind the cerebellum are two tubercles, which in the
anterior pair of the bigeminous eminences in proportion ray give origin to the fifth pair, and are very distinct in
to the testes or posterior pair. In the Rodentia the or- the pike, trout, salmon, and perch. These tubercles are
gan is distinguished for the large size of the nates, and peculiar to this class.
the want or superficial nature of the convolutions. In the The cavities in the interior of the optic eminences in
Ungulated division of animals, i. e. Pachydermata, Birds, Reptiles, and Fishes, are observed in the foetal
Ruminantia, and Solidungula, the brain is remark- brain of the Mammalia during its early growth,
able for the large size of the nates combined with the It is almost superfluous to mention, that, in the two
number and depth of the convolutions; while that of the warm-blooded classes, Mammalia and Birds, the brain,
Cetacea is remarkable for its height and breadth, and with its investments, fills completely the cranial and verte-
the want of olfactory nerves. It is further to be observed bral cavities. In the Reptiles, however, in which the brain
as a general distinction between herbivorous and carni- does not approach the cranial walls, we remark the first
vorous or zoophagous animals, that in the former the nates departure from this arrangement; and in the Fishes it is so
are larger than the testes, whereas in the latter the testes completely violated that the brain and chord occupy but a
are largest. Lastly, Man and the Quadrumana are the small proportion of the cranio-vertebral cavity; and be-
only animals which possess genuine olfactory nerves. In tween the former and the osseous walls there is a quantity
the other quadrupeds they are represented by the mam- of fine but very loose filamentous tissue, containing in its
miliary processes of the ancients; and in the Cetacea cells a large quantity of pellucid fluid. Though this ar-
they have not yet been unequivocally demonstrated. rangement gives this the appearance of a white jelly-like
The brain of Birds is at once recognised by consisting substance, it is not gelatinous, as is generally represented,
of six distinct tubercles, two representing the cerebral but merely a pellucid fluid, sometimes pale straw-coloured,
hemispheres, two representing the optic eminences, one occasionally with a reddish tint, contained in numerous
the cerebellum, and one the bulb of the chord. The communicating cavities of a tissue which appears to repre¬
hemispheres are void of convolutions, but the cerebellum sent the arachnoid of the warm-blooded animals.