Page:The New International Encyclopædia 1st ed. v. 14.djvu/440

* NERVOUS SYSTEM. 390 NERVOUS SYSTEM. seiul oil' various brauchcs aiul luiinnissures. The pallial commissures are uniti'il posteriorly. In the Puk'cypoda the nervous system is somewhat more comple.x, but it is always symmetrical, and consists of two ganglia, the cerebro-pleural, one on each side of the gullet, and united above by a commissure. Each ganglion gives off a nerve cord, which passes downward and backward to the bilobed pedal ganglion of the foot. Directly backward from the cerebral ganglia also proceed two other cords to the bilobed. fused, visceral ganglion, situated on the ventral side of the pos- terior adductor muscle: this ganglion sends out branches to the palps and mantle. The pedal ganglion innervates the foot, the viscera, the enteric canal, the heart, the gills, and a part of the mantle. The nervous system of gastropods varies in the different groups both in regard to the arrangement of the ganglia and the commis- sures. There is usually a pair of ganglia near together or some distance apart, which lie above the gullet, and give off the visceral nerve cords behind. The visceral cords pass eventually into a pair of ganglia, but only after they have passed through the pKniral ganglia. Pedal and buccal ganglia with their commissures also exist. The nervous system of annelids is well devel- oped and has a bilateral ami metameric arrange- ment. It consists of the bilobed cerebral ganglion or brain and of a double ventral chain of ganglia and commissures. The brain lies in the prosto- mium or a little farther back, as in the case of the earthworm. Branches are given off from it to the eyes, tentacles, and sensory epithelium. Two lateral commissures from it pass vent rally around the alimentary canal and unite below it into the double ventral nerve chain. One ganglion occurs in this nerve chain for each segment. In some forms the two halves of the chain are widely separated and are united by transverse commis- sures. The arrangement of the nervous system of arthropo<ls is essentially like that of annelids. There is a supraiesophageal ganglion or brain that gives olT commissures which pass downAvard and backward to unite with the first ganglion of the ventral nerve chain. Various degrees of con- centration of the nervous system occur through- out the Crustacoa and the insects. The higher Crustacea, as well as the insects, have a visceral nervous system. The brain of insects is rela- tively large and is lobed. VEUTKniiATKS. The nen-ous system of the higher vertebrates consists of a double chain of ganglia lying in the median line on the dorsal side of the body, which consists of the spinal cord and the brain. The segmental origin of the spinal cord is shown by the regular repetition of the spinal nerves that branch out from it in pairs, one on each side. The visceral or sympa- thetic system of vertebrates consists of a series of ganglia on each side of the vertebral column. Fibres connect this system with the spinal nnd cranial nerves. Xerves from the neural axis pass to all the various sense organs, muscles, and glands. The spinal cord is made np of two kinds of nervous matter, a superficial white layer and an internal H-slmped rod of gray matter. The cord merges gradually into the brain, which has a bilateral form an<i consists, in its sim- ph'st form, of thrif |)arts, the f(nebrain, mid- brain, and hind-brain. The two lobes of the cerebrum comprise the most anterior part of the brain. The mid-brain connects the fore and liiml brain. The hind-brain inchules the bilobed cere- bellum on its dorsal side and the pons Varolii and medulla oblongata on the ventral side. The brain is made up ot two kinds of nervous matter, the white and the gray. Its surface is nuich con- voluted, so as to give a greater surface for the gray matter which forms the outer coating. The cranial nerves arise from the lower surface of the brain. The brain of the Acrania (Amphioxus) is the enlarged anterior end of the neural axis and in it the central canal enlarges to form the ventricle. The ventricle opens dorsally to the exterior of the brain. Anterior to the opening there is a pocket known as the olfactory lobe. The brains of cyclostomes and teleosts re])resent a low stage of development in that they possess an unpaired prosencephalon whose dorsal wall, the corti cerebri, consists of a single layer of cells covered by the pia mater. In the larva of cyclostomes (Annnocetes) the metencephalon is the largest part of the brain. Selachians ])ossess a more complicated brain. They lead an active, ]ireda- tory life, and correspondingly the prosencephalon is relatively large and shows a slight furrow or division into two lateral parts or lobes. A prom- inent pair of bulbi olfa<'torii pass from in front to the nose. The thalamencephalon gives rise to two small optic thalami and there are other modifications in the rest of the brain which render it a more complex organ. There is a bundle of fil)res from each side of (he prosen- cephalon that probably corresponds to the cms cerebri of manunals. The brains of ganoids and dipnoans resemble those of amphibians in the absence of the lobi inferiores. In all three the cereliral hemispheres show a marked develoiiment. The cerebellum exists only as a small fold nt the anterior end of the medulla. In the Anura the optic lobes are well developed. In Reptilia. as in Amphibia, the cerebellum is small. A well-developed median eye-like struc- ture, the pineal eye, is present on the root of the mid-brain. The prosencephalon is the largest part of the brain. It is paired, and its surface is smooth. There is a large olfactory lobe nnd a well-developed corpus striatum. The avian brain is characterized by a considerable cranial flexure, by large cerebral lobes, by a cerebellum. In sagittal section the latter shows the arhor- vitae structure so evident in the brain of man. Externally the brain is folded transversely or convoluted. The manunalian brain exhibits in the did'ercnt grou]is a steady growth in complex- ity from the reptilian to the human condition. The cranial flexure is most pronounced in the mammalian brain. That the degree of development of the different organs within the group of mammals is dependent upon their functional aetivit.v can hardlv be doubted. Thus, the 'pyramids' are the motor nerves coming from the coilrdinnting centres of movement. These are relatively small in the dolphins, which possess no hinder extremities. The frontal lolie of the cerebrum, upon which probably many of the higher functions, sich ns spce<h, depend, are best developed in man: so, too. the olfactory lohes are reduced in man in correspondence with the nnlimentarv condition of their function in man. While the midbrain is more or less exposed in some of the lower mam- mals (e.g. marsupials, rodents, and insectivores), the hemispheres come gradually to cover the