EXCRETION. 485 



muscle at the vesic orifice. When the accumulating urine reaches a certain 

 volume, it gives rise to an intra-vesic pressure. When this pressure rises to 

 about 80 cm. of water the detrusor urincR acquires a certain degree of tension 

 or tonus. This is followed by rhythmic contractions of the detrusor urinse 

 which increase in extent and vigor as the urine continues to accumulate until 

 finally a general contraction develops, the force of which overcomes the 

 constricting influences at the bladder orifice and the fluid is discharged. 

 This action of the detrusor muscle is generally reinforced by the contraction 

 of the abdominal muscles. The latter portions of the urine are ejected 

 through the urethra by the rhythmic action of the accelerator urinae muscles. 



The Nerve Mechanism of Urination. The muscle mechanisms which 

 retain as well as expel the urine are under the control of the nerve system. 

 The nerves involved in the regulation of this mechanism reach the bladder 

 by two different routes, viz: (I) by way of the lumbar nerves and their con- 

 tinuations, the hypogastrics, and (II) by way of the sacral nerves and their 

 continuations in the pelvic plexus. The centers of origin of these special 

 nerve fibers are located in the spinal cord in the neighborhood of the third, 

 fourth and fifth lumbar segments. 



The lumbar nerves from the third to the fifth give off branches (pre- 

 ganglionic) which pass forward to the inferior mesenteric ganglion, around 

 the cells of which their terminal branches arborize; from the cells of this 

 ganglion new fibers arise (post-ganglionic), which descend through the hypo- 

 gastric nerves to the muscle coat of the bladder. 



The sacral nerves from the second to the fourth, give off branches which 

 emerge from the sacral foramina and then pass forward in the nervi erigentes 

 (pre-ganglionic) to small ganglia in the pelvic or vesical plexus around the 

 cells of which their terminal fibers arborize ; from these ganglia new fibers 

 arise (post-ganglionic) which are distributed also to the muscle coat of the 

 bladder. Afferent fibers pass from the mucous coat through the posterior 

 roots of the sacral nerves and reach the spinal cord centers from which 

 the efferent fibers for the muscle coat emanate. 



Though the origin, course and distribution of the nerves composing this 

 mechanism are fairly well known, their mode of action is somewhat obscure 

 and the results of experimentation not always in accord. According to v. 

 Zeissl stimulation of the peripheral ends of the divided hypogastric nerves 

 causes mainly a contraction of the sphincter muscles and a relaxation of the 

 detrusor muscle, while a stimulation of the peripheral ends of the divided 

 sacral nerves causes a vigorous contraction of the detrusor muscle and a re- 

 laxation of the sphincter muscles. The lumbar centers would therefore 

 cause a reception and a retention of the urine, and the sacral centers would 

 cause its expulsion. 



The expulsion of the urine is primarily a reflex act, though in the adult it 

 is subject to a variable amount of volitional control. When the accumulated 

 urine has reached a certain volume it causes, as previously stated, an 

 intra-vesic pressure, a muscle tonus, and slight rhythmic contractions of the 

 detrusor muscle. Coincidently nerve impulses are developed in the termin- 

 als of the afferent nerves in the mucous coat which are then transmitted to 

 the spinal cord centers and to the brain, where the characteristic sensation 

 and the desire to urinate arises. This desire is probably reinforced by 



