860 PHYSIOLOGY OF DIGESTION AND SECRETION. 



through the so-called nervi erigentes or pelvic nerves and the hypo- 

 gastric plexus. Stimulation of these nerves causes strong contrac- 

 tions of the bladder, sufficient to empty its contents. (3) Experi- 

 ments in lower animals indicate that the urethra, in addition 

 to the paths just described, receives fibers also from the pudic 

 branch of the sciatic plexus. According to Nawrocki and Skabit- 

 schewsky,* the spinal sensory fibers from the bladder are found in 

 the posterior roots of the first, second, third, and fourth sacral 

 spinal nerves, particularly the second and third. When these 

 fibers are stimulated they excite reflexly the motor fibers to the 

 bladder found in the anterior roots of the second and third sacral 

 spinal nerves. The relations of this nerve-supply to the act of 

 micturition is difficult to state. That the act is essentially a reflex 

 through the central nervous system is shown by the fact that 

 section of the nervi erigentes or of the posterior roots of the 

 sacral spinal nerves abolishes the act and leads to a distention of 

 the bladder. As far as experiments have gone they indicate that 

 the motor path for this reflex lies through the nervi erigentes or 

 pelvic nerves, while the afferent paths enter the cord in the pos- 

 terior roots of the sacral spinal nerves, coming through the pelvic 

 or the pelvic and pudic nerves. The hypogastric nerve apparently 

 does not enter into the reflex, but its inhibitory influence upon the 

 bladder may, by relaxing the tone of the musculature, provide a 

 mechanism for holding larger quantities of urine. So far as the 

 bladder is concerned there is evidence that the hypogastrics and 

 pelvic nerves are constantly in tonic action, one tending to relax 

 and the other to increase the tonicity of the bladder musculature, 

 forming thus balanced antagonists. An interesting feature of the 

 reflex is the part taken by the internal sphincter. During the 

 filling of the bladder this sphincter is in tone, at the time of empty- 

 ing its tone presumably is inhibited as a part of the reflex act. 

 Experiments indicate that stimulation of the pudic nerve causes 

 contraction of the sphincter, while stimulation of the pelvic nerve 

 causes relaxation. It is possible, therefore, that in reflex mic- 

 turition the pelvic nerves carry the motor impulses that cause 

 contraction of the bladder musculature and, at the same time, 

 through other fibers, inhibitory impulses that relax the sphincter. 

 The mechanism is, however, complex and needs further investiga- 

 tion.! 



Excretory Functions of the Skin. The physiological activi- 

 ties of the skin are varied. It forms, in the first place, a sensory 

 surface covering the body, and interposed, as it were, between the 



* "Archiv f. die gesammte Physiologic," 49, 141, 1891. 



t Consult Barrington, "Quarterly Journal of Exp. Physiology," 8, 33, 

 1914; Elliot, "Journal of Physiology," 35, 367, 1907; Stewart, "American 

 Journal of Physiology," 2, 182, 1899. 



