EXCRETION. 499 



is practically certain that the sweat-glands are also under the control 

 of efferent impulses passing to them from the special sweat centres 

 in the brain and spinal cord through special sweat nerves. Thus, if 

 the sciatic nerve be divided in a cat and the peripheral end be stim- 

 ulated, beads of sweat are seen to appear upon the pad of the correspond- 

 ing foot, although at the same time the blood-vessels are constricted or 

 while the aorta is pressed upon, whereas if atropin have been injected 

 previously to the stimulation, no sweat appears, although dilatation of 

 the vessels be present. Secretion of sweat, too, may be reflexly brought 

 about. 



The circulation of venous blood in the spinal bulb causes the sweating 

 of phthisis and of dyspnoea generally, by stimulating the sweat centre. 

 If the cat whose sciatic nerve is divided be rendered dyspnoeic, abundant 

 sweat occurs upon the foot of the uninjured, and none on the injured 

 side. The effect of heat in producing sweating may be both local and 

 general, and again, the various drugs which produce an increased secre- 

 tion of sweat do not all act in the same way; thus, there is reason for 

 thinking that pilocarpin acts upon the local apparatus, that strychnia 

 and picrotoxin act upon the sweat centres, and that nicotin acts both 

 upon the central and upon the local apparatus. 



The special sweat-nerves appear to issue from the spinal cord, in the 

 case of the hind limb of the cat by the last two or three dorsal and first 

 two or four lumbar nerves, pass to the abdominal sympathetic and from 

 thence to the sciatic nerve. In the case of the fore limb, the nerves 

 leave the cord by the 5th and 6th cervical nerves into the thoracic sym- 

 pathetic, and then join the brachial plexus, reaching the arm through 

 the median and ulnar nerves. 



It will be as well to repeat here the other functions which the skin 

 subserves. In addition to its excretory office, we have seen that it acts 

 as a channel for absorption. It is also concerned with a special sense, 

 viz., that of touch, to the consideration of which as well as to its func- 

 tion of regulating the temperature of the body we shall presently return. 

 It should be recollected, however, that apart from these special func- 

 tions, by means of its toughness, flexibility and elasticity, the skin is 

 eminently qualified to serve as the general integument of the body, for 

 defending the internal parts from external violence, while readily yield- 

 ing and adapting itself to their various movements and changes of 

 position. 



