208 EXCRETION BY THE KIDNEYS AND SKIN 



muscles compresses the bladder; after a short interval the 

 sphincter relaxes and allows the stream to pass out through the 

 urethra. When the act has been begun contraction of the blad- 

 der will suffice to nearly empty the organ, but complete evacu- 

 ation is finally brought about by a series of convulsive con- 

 tractions on the part of the muscles of the abdomen. 



The center controlling the reflex nervous phenomena of 

 micturition is opposite to the fourth lumbar vertebra in the 

 spinal cord. 



THE SKIN. 



Functions. The functions of the skin from a physical stand- 

 point are sufficiently apparent. It furnishes protection to the 

 underlying parts, preserves the general contour of the body, 

 affords lodgment for afferent nerve terminations, and thus estab- 

 lishes relations between ourselves and our surroundings. As an 

 organ of excretion it is very important, and in fact essential to life. 

 While various materials, such as urea and CO 2 , are thus dis- 

 charged from the body, their amount is more or less inconse- 

 quential, and it appears that it is the action of the skin as a 

 regulator of heat " excretion" which is vital. It furnishes one of 

 the three chief routes for the discharge of water from the body, 

 and it will be seen that it is largely through the output of water 

 that the output of heat is regulated. So necessary is the skin in 

 this respect that the covering with impermeable substances of as 

 much as half the body surface is followed by death. 



The skin excretions are contained in the products of the seba- 

 ceous and sweat glands. These products correspond altogether 

 to neither the secretions nor the excretions, and the sebaceous 

 glands have been described under the head of secretion. It is 

 to be remembered, however, that the sweat usually represents 

 part of the sebaceous as well as the sudoriparous secretion, 

 because the mixture of the two is a physical necessity. It is the 

 water of the sweat which is the most important excretion from the 



