900 CENTRAL NERVOUS SYSTEM 



may be a reserve power for .times of need. The gastrointestinal tract 

 is brought into activity by the vagus which thus contributes to the nu- 

 tritional processes of the body. When an animal has acquired food, cer- 

 tain rather specific reflexes take place through the bulbar outflow which 

 result in the psychic secretion of salivary and gastric juice and the re- 

 flexes of swallowing, and thus initiate the process of digestion. The 

 succeeding stages in the process can no doubt be executed by the intrinsic 

 mechanisms of the gastrointestinal tract, but activity of the vagus fibers 

 should be expected to counterbalance the inhibitory influence of the thorac- 

 ico-lumbar outflow and thus to reinforce the action of the gastrointestinal 

 muscles. 



The bulbar outflow brings about those reactions in the sphincters 

 and muscular walls of the bladder and cloaca which result in the dis- 

 charge of waste materials from these receptacles. Although these re- 

 sponses may occur automatically w^hen the spinal connections are de- 

 stroyed, under normal conditions these acts are made volitionally at 

 the convenience of the individual. In them a close cooperation occurs 

 between the action of voluntary and involuntary muscles in which affer- 

 ent impulses from these viscera play an important part. 



The mechanism for emptying the bladder illustrates this correlation 

 between voluntary and involuntary action. Reflex micturition is set 

 up when, through the accumulation of urine, the intervesical pressure 

 reaches 15 to 18 cm. of water. As the tension increases, rhythmic con- 

 tractions of the bladder musculature occur which increase in strength and 

 result in afferent impulses being set up in the pelvic nerves which pass 

 to the sacral cord and to the higher parts of the central nervous system. 

 Reflexes are thus set up through centers in the lower cord which result 

 in excitation of the sacral autonomic supply to the bladder, causing 

 a contraction of the bladder walls and inhibition of the sphincter. The 

 pressure within the bladder rises to 20 or 30 cm. and forces the urine 

 through the neck of the bladder and through the urethra. Normally 

 the emptying of the bladder is accompanied by a voluntary effort ini- 

 tiated by the afferent impulses which ascend to the cerebrum, and there 

 give rise to the sensations aroused by the distended vesicle as well as to 

 efferent impulses w r hich contract the respiratory and abdominal muscles, 

 so as to increase the intraabdominal pressure, and help squeeze the urine 

 out of the bladder. Reflex emptying of the bladder depends then on the 

 development of a certain tension in the vesicular muscle. It may be in- 

 hibited to a certain extent by voluntarily contracting the striated peri- 

 neal muscles which help in the closure of the urethra, either before or 

 during the reflex act. Micturition may be initiated before the bladder 

 is full enough to start the reflex, either volitionally or by virtue of im- 

 pulses arising through sensory nerves from other parts of the body. 



