VERTEBRATES. The nervous 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 consist: of the spina] cord and the brain. The segmental origin of the spinal is shown by the regular repetition of the spinal nerves that branch out from it in pairs. one on eaelt side. The visceral system of vertebrates consists of a series of ganglia on each side of the column. connect this system with the spinal and cranial nerves. Nerves from the neural axis pass to all the various sense organs, muscles, and gland,. The spinal cord is made up of two kinds of nervous matter. a superfieial white layer and an internal rod of gray matter. The cord merges gradually into the brain, which has a bilateral form and consists. I II its slur .1.4 Of three parts. the mid• brain. and 11. The two Obl, Of the cerebrum comprise the most anterior part of the brain. rim, hind brain. The includes the bilobed bellum on its dorsal side and the pons Varolii and medulla oblongata on the ventral side. The brain is made up of two kinds of nervous matter, the white and the gray. Its surface is much 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 represent a low stage of development in that they possess an unpaired prosencephalon whose dorsal wall, the corti eerebri, consists of a single layer of cells covered by the pia mater. In the larva of cyelostomes (Anunocet es) the metenccphalon is the largest part of the brain. Selaehians possess a more complicated brain. They lead an active, tory life, and correspondingly the is relatively large and shows a slight furrow or division into two lateral parts or lobes. A prom inent pair of bulbi olfactorii 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 Ivhich render it a more complex organ. There is a brindle of fibres from each side of the prosen cephalon that probably corresponds to the erns cerebri of mammals. Thp of ganoids and dipnoans resemble those of amphibians in the absence of the lobi inferiores. In all three the eerebral hemispheres show a marked development. The cerebellum exists only as a small fold at the anterior end of the medulla. In the Altura the optic lobes are well developed.
In P,eptilia, as in Amphibia, the cerebellum is small. A well-developed median eye-like ture. the pineal eye, is present on the root of the The proscncephalon is the largest part of the brain. It is paired. and its surface is smooth. There is a large olfactory lobe and a corpus striatum. The avian brain is elmracterized by a considerable cranial flexure. by large cerebral lobes. by a cerebellum.
In sagittal section the latter shows the arbor vitT structure so evident in the brain of man. the brain is folded transversely or convoluted. The mammalian brain exhibits in the different groups a steady growth in 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 activity can hardly be doubted. Thus, the are the motor nerves coming from the ee7Srdivaiing centres of movement. These are relatively I in the dolphins, which possess no hinder extremities. The frontal lobe of the upon whieh probably many of the higher funotions. such as speech, depend. are best developed in strum so, too, the olfactory lobes are redneed in man in correspondence with the rudimentary condition of their function in man. While the mid-brain is more or less exposed in some of the lower mam mals (e.g. marsupials. rodents. and inseetivores), the hemispheres come gradually to cover the entire brain. The mammalian brain is char acterized by the presence of large cunnissures, the corpus callosum and fonnix, which connect the hemispheres. The puns Varolii is another large commissure and connects the two lateral lobes of the cerebellum, and a number of other Lauds serve as connectives. En mammals the mid-brain is relatively smaller than in other vertebrates. The brain of Tertiary mammals was relatively small. The reduction was especially evident in the hemispheres. In many points of structure their brain, too, was reptilian in char acter. although the possessors were related to groups as high as ungulates.
Three main and well-defined types of nervous system may be determined in the animal king dom. There is first the radial type, such as has been described for the starfish. This consists of a ring of nervous tissue in the disk which surrounds the (esophagus and sends out a branch to each arm. This is the echinoderm type. An other type is the symmetrical, bilateral type of annelids and arthropods, to which the nervous system of mollusks may also be relegated. This type consists of a brain ganglion above the cesoph agus, with commissures that pass and below the digestive tract, where they unite ven trally with a chain of ganglia extending posteri orly. The pedal ganglion of mollusks repre sents the sulaesophageal ganglion. The rest of the ventral chain is absent in unsegmented mol lusks. The third type of nervous system is the bilateral type of vertebrates. Here the double nerve cord, segmental in origin, as is shown by the repetition of paired spinal nerves, lies on the dorsal side of the alimentary tract. Except in Amphioxus the vertebrate nerve cord widens anteriorly into a large and complex ganglion mass. the brain. The sympathetic system, which appears first in leeches and arthropods. is an almost independent nervous system, for while it is in direct union with the other nervous system, it is not at all under its control.