Page:The American Cyclopædia (1879) Volume XII.djvu/242

 234: NERVOUS SYSTEM nervous system is constructed presents two varieties, described by anatomists as the white or fibrous matter, and the gray, cineritious, or vesicular matter. The white matter is devel- oped in the form of fibres or filaments, bound together in fasciculi or bundles, which consti- tute what are called nerves ; their function is internuncial or conducting. The gray matter is accumulated in masses called centres or gan- glia ; their office is dynamic, originating the so- called nervous force. (For a more particular description of the microscopical anatomy and chemical analysis of nerve tissue, see BRAIN.) Examining the nervous system in its compara- tive development in the animal creation, we find four principal types exhibited in the four great classes radiata, mollusca, articulata, and vertebrata. We will briefly examine these in succession. 1. Nervous System of Radiata. In this class we find the simplest form of ner- vous system, and one which corresponds with the simplicity of the physical organization, the animal being formed by a series of similar parts. Thus in the asterias or five-rayed star fish we have a chain of five ganglia surround- ing the central mass, or body which contains the mouth and digestive cavity. These ganglia are united to each other by nervous arcs called commissures, and each sends into the ray or limb opposite to which it is placed nervous prolongations. 2. Nervous System of Mollusca. In these animals we have several separate and distinct organs, such as a digestive apparatus, a liver, gills, ovaries, and testicles, the male and fe- male functions generally coexisting in the same animal ; we have besides a muscular mantle and a foot or organ of locomotion. The necessi- ties of this physical organization are met by a corresponding arrangement of the ner- vous system. Thus in aplysia there is an anterior or cerebral ganglion resulting from the junction of two above the oesophagus ; this ganglion represents the brain, and is doubtless connected with whatever sensitive or volitional endowment the animal pos- sesses. From this small nerves pass to form the pharyngeal ganglion, which supplies in- nervation to the digestive apparatus ; from this two nerves pass backward to form the pedal ganglion, which also gives nerves to supply the mantle; and in the posterior part of the body there is an additional gan- glion, called the branchial, the nerves of which are distributed to the gills. 3. Ner- vous System of Articulata. In this class we observe that the body is divided into a certain number of similar segments, of which the anterior, called the head, pre- sents the greatest development. The ner- a vous system in this class corresponds to their anatomical structure. Thus in the common centipede, scolopendra, the nervous system consists in a double nervous cord run- ning along the ventral surface of the ani- mal, and presenting opposite to each articu- lation ganglionic enlargements ; these ganglia send off lateral prolongations, which unite with each other on the upper surface of the body, forming a series of circular commis- sures. At the head there is a ganglion at the Eoint of junction of the lateral prolongations rom the infra-cesophageal ganglia, called the supra-cesophageal ganglion, and from this ring or collar arise the optic nerves and the other nerves that are distributed about the head. In insects we find a somewhat more complicated arrangement of the same type of nervous sys- tem, and one whicji presents very important analogies to the nervous system of the verte- brata. It consists in a ganglionic nervous cord, but the ganglia are arranged so as to cor- respond with the anatomical division of the animal into three distinct cavities, the head, chest, and abdomen. In the head we have the ganglia presiding over the functions of special sense, prehension, and mastication ; in the chest those that control respiration and the locomotive organs, the legs and wings ; and in the abdomen those that are connected with the digestive and reproductive functions. 4. Ner- vous System of Vertebrata. The nervous sys- tem, as we find it developed in the mammalia, the last and highest class of the vertebrata, consists of an axis of nerve matter, called the cerebro-spinal axis, which is enclosed in the cavities of the cranium and spinal canal. This axis is constituted of ganglia and of longitudi- nal and transverse commissures, and gives off nerves which are distributed to the organs of special sense, of ordinary sensation and motion. This cerebro-spinal axis, however, with its centres, commissures, and nerves, forms but one portion of the nervous system in the ver- The Brain enclosed in its Membranes, and the Skull, ft, c. Convolutions of the cerebrum, d. Cerebellum. Medulla oblongata. /. Upper extremity of the spinal cord. g. Tuber annulare. A,, k. Central parts of the cerebrum. tebrata, that which presides over the purely animal functions of locomotion, respiration, sensation, and intelligence. A separate and distinct system, called the ganglionic or sym- pathetic, controls the functions of organic or