Page:The American Cyclopædia (1879) Volume V.djvu/181

 COMPARATIVE ANATOMY 177 in a great part of its extent by an immense number of bundles of nervous fibres, of a vastly greater bulk than the cord they surrounded. Where the pectoral fins are much developed, as in the rays and flying fishes, the cord is enlarged at the origins of the nerves; in the torpedo and electrical eel, nerves are freely distributed upon the thin membranous lami- nae which constitute the electric apparatus, and act as conductors, if not as generators, of this force. The cord is composed of an ex- ternal white or tubular portion, and an internal gray or vesicular matter, the reverse being the arrangement in the brain. In the amphioxus, the simplest vertebrate, the cord, with its nerves on each side, forms the whole nervous system ; but in fishes generally the cerebral ganglia, with the nerves of special sense, constitute a distinct brain. The olfactory lobes, by some considered the representatives of the cerebral hemispheres in man, from which the nerves of smell arise, are well developed, and in the sharks are four instead of two, the usual num- ber. The optic lobes, behind these, homolo- gous with the tubercula quadrigernina, are larger than the other parts of the brain, and are in proportion to the development of the optic nerves which arise from them and the perfection of the sense of sight. In the blind fish of the Mammoth cave of Kentucky (am- blyopsis spelaus), in which the eyes are rudi- mentary, there appears to be an exception to this law; though he could detect no optic nerves, Prof. Wyman found the optic lobes of good size, though less than in the allied fishes. Between the olfactory and optic lobes are the true cerebral hemispheres, largest in the sharks ; behind the optic lobes is the cerebellum, which comparative anatomy shows to preside over the coordination of the movements, largest in the active sharks. In the peretmibranchiate am- phibia the brain and nervous system are very much like those of fishes ; in the genera which undergo metamorphosis the changes from the fish-like to the reptilian brain are rapid and remarkable, the hemispheres becoming enlarged and the spinal cord shorter as the tail disap- pears. In frogs, there are found attached to each spinal nerve, before the division into sen- sory and motor roots, vesicles containing a white, chalky, crystalline substance; from their constant presence, they are considered essen- tial parts of the nervous system ; nervous fila- ments have been traced into the interior of these vesicles; the chalky matter resembles that found in the vestibule of the ear. In true reptiles the brain is more developed and nearly fills the cranial cavity ; the hemispheres are in- creased in size, and the cerebellum is large in proportion to the activity and complexity of the movements ; the nerves are large compared with the centres, and the sympathetic system is more closely connected with the blood vessels. In birds the parts of the brain are no longer on the same plane, are larger, more complicated, in- dicating more intelligence and more activity. In the spinal cord there is an enlargement where the nerves are given off to the wings, and an- other where those of the legs take their origin ; that these are not proportional to muscular force, but rather to sensibility, is shown by the fact that the latter is the larger ; though the wings are more muscular, they are, from their feather covering, less sensitive to external im- pressions than the legs ; this is more clearly shown in the mammalian bat, where the en- largement corresponding to the exceedingly sensitive wings is by far the greatest. In mam- mals the cerebral and cerebellar hemispheres reach their highest development, the for- mer gradually covering over the latter, and the convolutions becoming more numerous and complicated up to man ; the commissures more intimately connect their different por- tions; the spinal cord is larger in propor- tion to the size of the body, but smaller when compared with the brain ; and the sym- pathetic system is more extensively distribu- ted. Organs of Special Sense. By means of the organs of special sense, placed at the ex- tremities of the cerebro-spinal nerves, and generally near the entrance of the alimentary canal, animals are brought into relation with external objects ; an impression communicated to the outer surface is transmitted to the sen- sorium along the sensory fibres of nerves, and there causes the phenomena of smell, vision, taste, hearing, or general sensation, with their resultant motor acts from the transmission of the nervous influence along the motor fibres ; in all the vertebrates there are organs set apart for this purpose, the nose, the eyes, the ears, the tongue, and the skin. The sense of touch, residing in the skin, is the most universally diffused, and is capable of answering most practical purposes of the other special organs; and indeed the senses of taste and hearing seem to be little else than modifications of the sense of touch ; the senses of smell and vision depend upon the influence of such delicate changes in the surrounding air that we can hardly comprehend them except through their effects. The skin consists essentially of two layers, the cuticle and the true skin ; the sur- face of the latter is the seat of sensibility, and is provided with numerous papillaa into which the nervous loops enter ; the cuticle is made up of nucleated cells, becoming dry externally and falling off in the form of thin flakes. In fishes the body is generally covered with scales, not fit for receiving tactile impressions ; some of the siluroids, as the horn-pout, have fleshy barbels attached to the lips, into which nervous filaments may be traced ; their skin is lubricated with a viscid mucus poured out through numerous tubes. In the amphibia the skin is soft and yielding, and quite sensitive ; it is well known to be in this class an impor- tant accessory organ of respiration. In ser- pents and lizards the tongue and the tail are the principal organs of touch. In birds the tactile organs are the bill, the cere, and vari-