596 



THE NERVOUS SYSTEM 



effects of which are likely to lead to death unless the operated animal is 

 carefully attended. But these effects are slowly recovered from, and man, 

 as well as the lower mammals, soon regains the vascular tone. The general 

 tonus of the muscular system, which is lost at first, is also regained. 



In this partially recovered condition man, and such animals as the cat, the 

 dog, and the monkey, perform certain of the lower functions with a re- 

 markable degree of perfection. Of course these functions are under constant 

 co-ordinative regulation and control in the normal animal, but experiments 



FIG. 370. Scheme of the Relation of the Posterior Root Fibers upon Entering the 

 Cord. A, The branch of the dorsal root fibers upon entering cord; B, terminal arborization 

 about cell bodies of the cord; DR, axones of the dorsal root; B, their ascending and descend- 

 ing branches; C, collaterals. (After Cajal.) 



and observation have shown that much of such activity really is a primary 

 function of the cord. Of these activities the following may be especially 

 mentioned: Muscular tonus, general reflexes, the special reflexes of mic- 

 turition, defecation, erection and the sexual reflex, and parturition, some 

 of which will be briefly discussed. 



The Center of the Tone of Muscles. The tonic influence of the spinal cord 

 on the sphincter ani and sphincter urethras will be presently mentioned. The 

 cord maintains these muscles in permanent tonic contraction. The condition 

 of the sphincters, however, is not altogether exceptional. Their contraction 



