ORGANS of SENSATION.
THE organs of sensation exhibit greater varieties than those of motion. The lowest classes of animals, zoo phytes, and pay/vs, have no visible marks of a nervous system, though it is more than propable that they are capable of feeling, if not of tasting. All the classes of animals above these, however, have sensitive organs more or less perfect. All of them have nerves, most of them ganglia, and a great many possess a brain.
In no animal is the brain found so large in proportion to the nerves which proceed from it, as in man, though many of the MAMMALIA have a brain very similar in other respects to the human. Of these, the horse seems to have the largest brain ; but Soernmering has found that the largest horse's brain which he has examined is scarcely more than half the size of the smallest which he has seen in an adult man. Yet the nerves arising from the former were at leastten times larger than those from the latter. See Soemmering De Basi Encephali. In a great many mammalia the cerebellum is separated from the cerebrum by a bony instead of a membranous partition. This portion of the encephalon is also in most quadrupeds larger in proportion to the cerebrum than in man, and the same is the case with the medulla oblon gata.
The difference between the cerebrum and the medulla oblongata is most remarkable in the cctacea, especially in the dolphin, whose brain is fifteen times larger than the nzedulla oblongata.
It is remarkable that the pineal gland in the brain of quadrupeds has very little of that sandy substance which is so generally found in the brain of man.
In the brain of BIRDS there are several remarkable peculiarities. There are no circumvolutions like those of the intestines, as in the brain of man and most main malia, and the parts called fornix, corpus callosum, and corpora striata are wanting. In birds also, the situation of the cortical and medullary parts is reversed, the for mer being the central and the latter the peripheral sub stance. The ventricles also are on the outside of the medullary substance. According to Cuvier, the brain of these animals consists of six tubercles, viz. two hemi spheres, two optic thalami, a cerebellum, and medulla ob longata, all visible exteriorly.
The brain of REPTILES and SERPENTS is small and simple, consisting of five tubercles, as there is no evi dent medulla oblongata. In these animals the spinal marrow is of prodigious size when compared with the brain.
One of the most remarkable circumstances in the brain of FISHES, is that it does not fill the cavity of the skull; and that in general it is surrounded by a salt, fatty fluid, contained within a loose cellular membrane. In these animals too, the optic nerves evidently cross each other.