424 EXCRETION 



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

 about reflexly. 



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

 phthisis and of dyspnea generally, by stimulating the sweat center. If the 

 cat whose sciatic nerve is divided be rendered dyspneic, 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 secretion of sweat do not all 

 act in the same way; thus, there is reason for thinking that pilocarpine acts 

 upon the local apparatus, that strychnine and picrotoxin act upon the sweat 

 centers, and that nicotine 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 to four lumbar nerves, pass to the abdominal sympathetic, and from 

 thence to the sciatic nerve, the general course of the autonomic nerves 

 for this region. In the case of the fore limb, the nerves leave the cord 

 by the first to the sixth dorsal nerves, pass into the thoracic sympathetic, 

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

 ulnar nerves. 



It will be as well to restate here the other functions which the skin sub- 

 serves. In addition to its excretory office, we have seen that it acts as a 

 channel for absorption. It is also concerned with the special senses, that of 

 touch and temperature, to the consideration of which as well as to its function 

 of regulating the temperature of the body we shall presently return. By its 

 general impermeability it prevents the loss of moisture of the body by direct 

 evaporation from the tissues. It should be recollected, however, that apart 

 from these special functions, by means of its toughness, flexibility, and elastic- 

 ity, the skin is eminently qualified to serve as the general integument of the 

 body, for defending the internal parts from external violence, while readily 

 yielding and adapting itself to their various movements and changes of 

 position. 



LABORATORY EXPERIMENTS IN EXCRETION. 

 PHYSIOLOGICAL REACTIONS. 



i. The Relation of Blood Flow through the Kidney to the Secre- 

 tion of Urine. Properly to check this experiment one should make three 

 determinations: i, the general blood pressure; 2, the volume of the kid- 

 ney; 3, the amount of urine secreted. Anesthetize a dog and arrange the 

 apparatus for taking the blood pressure as directed in experiment 19. Pre- 

 pare a renal onkometer, see figures 301 and 302, and an onkograph for re- 

 cording the variations in the volume of the kidney. The renal onkometer 



