250 



NUTRITION 



FIG. 135 



it may be postponed, but by irritation of the nerves in its walls the ever- 

 distending viscus gradually increases the sense of discomfort until it 

 perhaps becomes actual pain. 



The forces acting in micturition are the contraction of the bladder's 

 muscular walls, the passive elasticity of the viscus, the weight and 

 pressure of the viscera above it, and the weight of the urine in the bladder. 

 The first two of these tend continually to lessen the size of the organ 



much as is the case with the stomach. 

 These four forces combined, and aided 

 in cases of haste by voluntary con- 

 traction of the rectus abdominis, etc., 

 quickly overcome the cohesion of the 

 urethra's walls and the urine passes 

 out, under a pressure of about 100 

 mm. of mercury or more. The urine 

 left in the male urethra is then ex- 

 pelled by the bulbocavernosus and the 

 leva tor ani muscles. 



The nerves concerned seem to come 

 from the second, third, fourth, and 

 fifth lumbar vertebral segments, but 

 the sensory (afferent) and the in- 

 hibitory (efferent) impulses pass up 

 and down the cord's lateral columns 

 to the cortex cerebri, while some 

 may go by way of the sympathetic. 

 The inhibitory influence relaxes the 

 sphincter. The mechanism may act 

 quite independently of the central 

 nervous system, directed probably by 

 influences from local ganglia or a 

 nervous reticulum. Friedman places 

 a supposed micturition-center in the 

 upper third of the posterior central 

 convolution just behind the center 

 for the arm's action. The bladder's 

 contractions are rhythmic in nature, 

 following each other at intervals 

 averaging about fifty seconds (Sher- 



rington), in a way somewhat similar to the contractions of the uterus 

 and the spleen. The difference between the neural mechanism in the 

 adult and that in the infant may very well be largely functional, and 

 consist in a development of voluntary control over the smooth muscle 

 of the bladder. This is seen frequently in almost all the smooth 

 muscles of the body, and is not very rare even in case of the heart. 

 The bladder is one of the most sensitive of the organs to nervous 

 excitement and influences of many sorts, chemical as well as nervous. 



The innervation of the kidney and of the 

 bladder: K, kidney; B, bladder; DXIII, 

 thirteenth dorsal root (dog); pn.g, vagus; 

 n.sp, splanchnics (great and small); gg.ms, 

 superior mesenteric ganglion; gg.hyp, hypo- 

 gastric ganglion or plexus; n.er., erector 

 nerve. (Morat.) 



