5<=> 



CLASSIFICATION 



■A 



the head-quarters of the life of the individual; and of numerous 

 branching cords, the nerves, which connect up the central organs 

 with all parts of the body. The central organs consist of brain 



and spinal cord, elaborately sheltered 

 within the axial skeleton (fig. 26), and 

 of a more subsidiary part lodged in the 

 body cavity close against the backbone, 

 and known as the sympathetic nervous 

 system. 



The Spinal Cord. — It is best to begin 

 a consideration of these organs with the 

 spinal cord or marrow, a soft cylindrical 

 mass contained in the backbone, and 

 continuous at one end with the brain, 

 while at the other it tapers to a thread. 

 This cord is protected by investing mem- 

 branes, and in a cross -section (fig. 27) 

 presents a somewhat singular appearance, 

 due to its being made up of two sorts 

 of nervous matter, white and grey, the 

 former being external while the latter 

 constitutes a sort of core, which in the 

 section looks like a couple of cres- 



Fig. 26. — Position of Brain and 

 Spinal Cord 



A, Right cerebral hemisphere ; b, 

 cerebellum: c, upper end of medulla 

 oblongata d d, spinal cord with be- 

 ginnings of spinal nerves; ee, pro- 

 cesses of vertebrse; F G H, last verte- 

 brae of neck, chest, and loin regions; 

 I, sacrum. 



Fig. 27.— Cross section of the Spinal Cord, dorsal side below. Partly 

 diagrammatic. Magnified 



NC, Groups of nerve-cells in grey matter; remaining capital letters indi- 

 cate regions of white matter; sp, a spinal nerve, formed by union of dorsal 

 (posterior) and ventral (anterior) roots, fr and ar; g, ganglion on dorsal 

 root. 



cents placed back to back. A further point of importance 

 is the presence of a minute central canal, so that the cord 

 can be regarded as a tube with small bore and very thick 

 walls, somewhat as is the case with the stem of a thermo- 



