ARRANGEMENT OF THE NERVOUS SYSTEM. 



cervical nerves which supply the arms, and the sacral nerves which 

 supply the legs, are larger than those given off in other regions ; and 

 the cord itself presents two enlargements corresponding with the 

 origin of these nerves, namely, the cervical enlargement, which is the 

 source of the nerves for the upper limb, and the lumbar enlargement, 

 which gives off the nerves for the lower limb. 



The origin of the spinal nerves on each side of the cord is by two 

 sets of fibres, forming anterior and posterior roots. The anterior 

 root (Fig. 97, d) emerges from the cord opposite the anterior horn 

 of gray matter. The posterior root (e) originates at a point corre- 

 sponding with the posterior horn of gray matter. Both roots are 

 composed of numerous fibres, united in parallel bundles. The posteriory 

 root is distinguished from the anterior by the presence of a small rounded . 

 mass of gray matter, or ganglion, beyond which the two roots unite 

 in a common trunk. 



The white substance of each lateral half of the eord is thus divided 

 into three portions or " columns ;" so called because the nerve fi ] 

 composing them run, for the most part, in a longitudinal direction. The 

 portion included between the anterior median fissure and the anterior 

 nerve roots is the anterior column; that between the anterior and 

 posterior nerve roots is the lateral column ; while that between the 

 posterior nerve roots and tin 1 posterior median fissure is the posterior 

 column. As the posterior median fissure penetrates the cord quite to 

 the gray commissure, the posterior columns appear entirely separated 

 from each other in a transverse section : 

 while the anterior columns are connected 

 by the white commissure above mentioned. 



The brain, or " encephalon," is that por- 

 tion of the cerebro-spinal mass contained 

 in the cranial cavity. It consists of various 

 deposits of gray substance, connected with 

 each other and with the spinal cord by 

 transverse, oblique, and longitudinal tracts 

 of nerve fibres. The number and size of its 

 nervous centres vary in different animal - 

 cording to their general bodily organization, 

 and the special development of their ner- 

 vous functioi 



In fish and reptiles the nervous centres 

 of the brain are so distinctly separated, and 

 of such moderate size, that they are often 

 designated as ''ganglia." In the alligator 

 (Fig. 98) there are five pairs of these gan- 

 glia, arranged in a double linear series. The 

 first are the olfactory ganglia ( M ) which distribute their nerves to 

 the olfactory membrane, and are connected with the rest of the brain 

 by slender longitudinal tracts of white substance. The next pair (.,_,,), 



FIG. 98. 



LLIGATOR,!, onactu- 

 ry Ganglia. 2. Hemispheres, o. 

 Optic Tubercles. 4. Cerebellum. 

 .1. Medulla Oblougata. 



