172 SPECIAL MUSCULAR MECHANISMS 



the urethra by the elasticity of the parts and perhaps also 

 by a tonic contraction of the internal sphincter of the bladder. 

 As it increases in amount and a desire to micturate arises, 

 the sphincter of the urethra is voluntarily contracted to pre- 

 vent the escape of urine, and the back flow through the ureters 

 is prevented by their oblique entrance. Micturition is initiated 

 voluntarily by a relaxation of the sphincter urethrse. The 

 walls of the bladder contract, driving the liquid out forcibly. 

 This may be aided by a closure of the glottis and a contrac- 

 tion of the abdominal muscles. In the male the last portions 

 are ejected in spurts by the contractions of the bulbocavernosus 

 muscle. The tone of the bladder is continually undergoing 

 changes, so that the pressure of the urine varies independently 

 of the quantity present. This explains why a desire to mic- 

 turate, if not satisfied, may pass off. The centre of micturition 

 has been located in the lumbar portion of the cord, between 

 the second and fifth lumbar spinal nerves. The bladder has 

 a double nerve supply. 



1. From lumbar nerves passing through the inferior mesen- 

 teric ganglion and hypogastric nerves. Stimulation of these 

 causes a feeble contraction. 



2. From sacral spinal nerve fibers contained in the nervus 

 erigens. Stimulation of these causes a strong contraction 

 (Fig. 8). 



Parturition. This process is inaugurated by painless, rhyth- 

 mical peristaltic waves that sweep over the upper segment 

 of the uterus. These contractions increase in intensity and 

 duration until they give pain, which is intensified by resistance 

 ahead or may be absent altogether if the child is small and the 

 canal large and free. The pain is at first confined to the uterus, 

 but later spreads up into the abdomen and down into the thighs. 

 Contractions in the human being involve only the upper por- 

 tions of the uterus, the lower segment and cervix remaining 

 passive. When the membrane which precedes the fetus in 

 its passage through the os uteri bursts, there is for a time a 

 cessation of uterine contractions (owing to the considerable 

 reduction in the bulk of the uterine contents), but they are 

 soon renewed with increased vigor, aided by forcible contrac- 

 tions of the abdominal muscles and by forcible expirations 

 with closed glottis. By these means the head of the fetus is 



