396 THE EXCRETIONS OF THE BODY 



which has induced Loulanie to describe the death of these animals as death 

 by inanition. 



The quantity of sweat excreted daily is variable. It depends chiefly upon 

 the requirements of heat regulation. The greater the quantity of sweat se- 

 creted, the greater is the absolute quantity of solid constituents, among 

 which urea is of special importance. Ordinarily the output of urea in 

 the sweat is negligibly small, and yet as already observed (page 89), under 

 certain circumstances it may become considerable. 



B. THE EXCRETORY PROCESS 



In view of the importance of sweat in regulating the temperature of the 

 body, it is but natural to assume that the action of the sweat glands is under 

 the control of the central nervous system. This is confirmed by experiment. 



Stimulation of the cut sciatic nerve or of the brachial plexus in a cat pro- 

 duces in a short time large drops of sweat on the balls of the foot (Goltz). This 

 production of sweat is an actual secretion and not a filtration from the blood, 

 for: (1) a powerful secretion can be evoked by stimulation as much as twenty 

 minutes after amputation of a leg (Kendall and Luchsinger) ; (2) secretion 

 occurs when the pressure of the surrounding air is higher than that of the aortic 

 blood (Levy-Dorn) ; (3) it does not occur without stimulation when a paw is 

 subjected to a low air pressure; and (4) it is prevented by very small doses of 

 atropine, despite the strongest nerve stimulus. 



The sweat fibers for the fore paw of the cat have been found in the median 

 and ulnar nerves, for the hind paw in the sciatic. It appears however that 

 most of them do not come directly from the spinal roots of these nerves, but 

 that they first traverse the sympathetic paths (thoracic or abdominal trunk) 

 before they join the nerves to the extremities. The sources of the sweat fibers 

 in the abdominal sympathetic are the three lower thoracic and the four upper 

 lumbar roots; those of the fore paw spring from the fourth thoracic root. 



Sweat centers are present in the spinal cord; for if the cord of a young 

 cat be cut at the level of the fourth thoracic root, secretion can be obtained 

 on the hind paws by the influence either of heat or of dyspnoea. Considering 

 the importance of sweat in heat regulation, it is very probable that a general 

 sweat center is present in the medulla, although we know nothing definite 

 about it at this time. 



Secretion of sweat is induced by psychical stimuli (fear, etc.), by heat, 

 asphyxiation, and reflex effects, as well as by various poisons. Among the 

 latter, pilocarpine is especially worthy of mention, for it has the power to 

 produce sweat even when the secretory nerves are cut. 



The effectiveness of a stimulus applied to the secretory nerves depends 

 mainly upon the temperature of the glands. When very cold, no effect at all 

 is produced, although at a body temperature of 22-28 C. the glands of a cat's 

 foot can be made to secrete by psychic excitation, by reflex action or by asphyxia- 

 tion. On the other hand heat produces secretion of sweat even in case the spinal 

 cord is severed at the ninth thoracic root and all the posterior roots of the sev- 

 ered cord are cut i. e., heat, like asphyxiation, has a direct stimulating effect 

 upon the sweat centers. 



