80 



ANATOMY FOE NUKSES. [CHAP. VII. 



As a result the impulse is distributed over an area supplied by 

 several sympathetic neurones. Similarly, sensory impulses, 

 originating in any part of the area supplied by a particular 

 group of sympathetic neurones, may be transmitted to a single 

 afferent dendrone which connects with the axones of several 

 sympathetic cells. These relations can best be understood by 

 studying the accompanying diagram (Fig. 68). 



The spinal cord and spinal nerves. The spinal cord is a 

 column of gray and white soft substance, extending from the 

 top of the spinal canal, where it is continuous with the brain, to 

 about the second lumbar vertebra, where it tapers off into a fine 



thread. Before its 

 termination it gives 

 off* a number of 

 fibres which form a 

 tail-like expansion, 

 called the cauda 



FIG. 68. DIAGRAM SHOWING THE RELATION OF 

 THE CEREBRO-SFINAL TO THE SYMPATHETIC NEU- 

 RONES. A, a medullated fibre, axone, or dendrone, 

 coming from cerebro-spinal system and dividing into 

 numerous branches on reaching a sympathetic ganglion. 

 These branches connect with those of the cells, B, B, in 



eqmna. 



Like the brain, 

 the spinal cord is 

 protected and nour- 

 ished by three 

 membranes. These 

 membranes have 

 the same names and 



the ganglion, and these cells send their non-medullated practically exercise 

 fibres, axones, or dendrones, to supply the viscera, ^g same functions 



as those enveloping 



the brain (for description of which see page 85). The outer 

 membrane is not attached to the walls of the spinal canal, being 

 separated from them by a certain quantity of areolar and 

 adipose tissue, and a network of veins. Therefore, the spinal 

 cord does not fit closely into the spinal canal, as the brain does 

 in the cranial cavity, but is, as it were, suspended within it. 

 It diminishes slightly in size from above downwards, with the 

 exception of presenting two enlargements in the cervical and 

 dorsal regions, where the nerves are given off to the arms and 

 legs respectively. It is usually from sixteen to seventeen 

 inches (406 to 432 mm.) long, and has an average diameter of 

 three-fourths of an inch (19 mm.). The spinal cord is almost 



