Page:The American Cyclopædia (1879) Volume XII.djvu/243

 1STERVOUS SYSTEM 235 d vegetative life; its centres are located in the visceral cavities of the body; they are con- nected with each other by longitudinal and transverse commissures, and send filaments to all the viscera. The cerebro-spinal axis, as it is developed in man, may be briefly described as follows. The principal ganglion is placed at the su- perior extremity of the axis, and is called the brain or en- cephalon. This is enclosed in the cavity of the cranium, and is enveloped by three membranes, the pia mater, the arachnoid, and dura ma- ter. The encephalon consists of a series of ganglia con- nected with each other, and designated, counting from before backward, as: 1, the olfactory ganglia ; 2, the ce- rebrum or hemispheres; 3, the corpora striata; 4, the optic thalami; 5, the tuber- cula quadrigemina ; 6, the cerebellum; 7, the ganglion of the tuber annul are ; and 8, the ganglion of the medulla oblongata. The cerebrum and the cerebellum present a convoluted surface; the others are smooth and irreg- ularly rounded. The supe- rior aspect of the brain pre- sents a deep longitudinal fis- sure, which divides the cere- brum into two lateral halves, called the cerebral hemi- spheres; these hemispheres, however, are united at their middle by a transverse com- missure, designated as the corpus callosum. Upon its inferior surface each hemi- sphere is seen to be divided into an anterior, middle, and posterior lobe. The remain- der of the axis, called the spinal cord, is a cylinder of nerve matter, extending through the entire length of the vertebral canal, where it is enveloped by a prolonga- tion of the membranes of the brain. Upon the ante- rior and posterior aspects of the spinal cord are two lon- gitudinal fissures, which in- dicate the two lateral halves of which the organ is com- From the brain and spinal cord there arise 44 pairs of nerves, perfectly symmetri- cal in their course and distribution. These nerves are distinguished as cranial and spinal. The Spinal Cord and Nerves, posterior view. a. Upper extremity of spinal cord. b. Lower extremity. c. Cervical portion. d. Dorsal portion. e. Lumbar portion. The cranial nerves, of which .there are 12 pairs, are distributed mainly to the organs of sense, and to those of respiration, voice, and speech; they emerge from the skull by the several openings at its base. The spinal nerves, of which there are 32 pairs, arise from the cord by two roots, one from the anterior and the other from the posterior portion. They es- cape from the spinal canal through the inter- vertebral openings, and are distributed prin- cipally to the voluntary muscles and the in- teguments of the neck, body, and extremities. The cerebro-spinal axis, or the nervous system of animal life, as it is called in contradistinction to the sympathetic system, or that of organic life, presents certain peculiarities in the several classes of vertebrata which deserve to be briefly mentioned. As we descend in the series of vertebrate animals, we are struck with the dif- ference in the relative size and development of the brain and spinal marrow. The annexed table will show the proportion which the one bears to the other in weight in the four classes : CLASS. SPECIES. Proportion of Pisces. Lamprey 100 750 100 180 100 80 100 22 Reptilia Triton. . Aves Pigeon Mammalia. Mouse The following table shows the relative propor- tions of the body and brain in the same classes : CLASS. SPECIES. Proportion of brain to body. Pisces Lamprey. 100 142 500 Eeptilia . Turtle .. 100 454,500 Aves Pigeon . . 100 9,100 Mammalia Mouse 100 8,500 This diminution in the size of the brain in the lower vertebrata is observed chiefly in the cerebral hemispheres, the centres of the intel- lectual faculties. A few words will describe some of the more important anatomical differ- ences in the cerebro-spinal axes of the three lower classes of vertebrata. The brain of birds presents a perfectly smooth surface, and is composed of six masses, viz., beginning ante- riorly : the two lateral hemispheres, the optic tubercles, the cerebellum, and the medulla ob- longata. The cerebellum is compressed later- ally and convoluted transversely. The braia of birds has in common with reptiles and fishes certain peculiarities never observed in the brain of mammalia. They are : 1, the absence of the corpus callosum or transverse commissure of the cerebral hemispheres ; 2, the separation of the optic tubercles from the hemispheres ; and 3, the existence of two ventricles in the optic tubercles. In reptiles, as in birds, the brain is without convolutions. The optic tubercles, instead of being smaller, are sometimes larger than the cerebral hemispheres, and placed as usual behind them. The cerebellum is behind
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