236 



PHYSIOLOGY FOR DENTAL STUDENTS. 



matic and abdominal muscles, the effect of which is to reduce the 

 size of the bladder cavity and to expel the urine with pressure, 

 through the urethra. 



The act is under nervous control, the motor nerves being de- 

 rived from nerve cells found in the lumbar region of the cord. 

 The stimuli here produced co-ordinate the muscular movements 

 of the act. The afferent or sensory stimuli which initiate the 

 act are excited by the distention of the bladder, or by the pass- 

 age of a few drops of urine into the first portion of the urethra. 

 These stimuli pass to the center in the cord and are returned to 



Vena cava 



Aorbe> 



U reWhro. 

 Fig. 38. Diagram of urinary system. 



the muscles of the bladder also causing the sphincter, which closes 

 the bladder, to be relaxed. In the voluntary act the motor nerves 

 are stimulated by impulses from the higher centers. 



The Function of the Skin. 



The skin serves a double function, that of protecting the body 

 from the outside environment, and that of excreting essential 



