THE PILO-MOTOR NERVES TO THE SKIN 447 



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 belong to the thoracid autonomies or sym- 

 pathetic nerves, and issue from the spinal cord with this outflow, page 

 636. The fibers join the chain ganglia by way of the white rami and 

 are thence distributed to all parts of the body. In the case of the hind 

 limb of the cat the sweat nerves arise from the cord by the last two or 

 three dorsal and first two to four lumbar nerves, pass to the sympathetic 

 chain, and from thence to the sciatic nerve, following 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 roots pass into the 

 thoracic sympathetic and then join the brachial plexus, reaching the arm 

 through the median and ulnar nerves. 



The Pilo-motor Nerves to the Skin. Small groups of smooth mus- 

 cle fibers are found generally distributed in the skin. They are attached 

 to the hair follicles and by their contractions produce erection of the hairs 

 or in man the roughness called goose-flesh. These muscles are innervated 

 by nerve fibers that belong to the thoracic autonomies. The origin and 

 path of distribution corresponds closely with that of the sweat 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. 



