82 PHYSIOLOGY 



the muscles in the walls of the blood vessels are con- 

 trolled by the vaso-motor nerves. These are very sen- 

 sitive to changes of temperature. When a tissue is act- 

 ive chemical changes take place which set free heat. 

 This heat stimulates the vaso-motor nerves, the nerves 

 convey the stimulus to the muscles in the walls of the 

 blood vessels; the muscles relax, the vessels dilate, and 

 more blood passes to the tissue. The blood brings food 

 materials to replace the protoplasm that has split up, 

 takes away the waste that has been formed in the 

 process, and distributes the heat through the body. 

 When the tissue becomes quiet less heat is set free, the 

 nerves again convey a stimulus to the fibers, they con- 

 tract and the supply of blood is decreased. The tone, 

 or sustained activity, of the blood vessels is therefore 

 very important not only because it aids the tissues 

 to get the amount of blood they need when they 

 need it, but because it helps to regulate the body tem- 

 perature. 



Relation to the Body Temperature. — The tempera- 

 ture of warm-blooded animals is self-regulating and 

 constant, although chemical actions always taking place 

 in the body are continually giving rise to heat. This 

 continual setting free of heat would cause the tempera- 

 ture of the body to increase were it not for the vaso- 

 motor control of the blood vessels that go to the skin. 

 As blood flowing through the skin is rapidly cooled 

 through contact with the air, the amount of blood 

 brought to the skin in a unit of time controls the amount 

 of heat given off by the blood in a unit of time. 



The least variation in the amount of heat present in 



