ANIMAL HEAT. 367 



central part of the gland being squeezed on to the surface of the skin 

 whenever the hairs are erected by the contraction of the arrector pili 

 muscles. The latter are composed of unstriated muscular fibres, and 

 are attached to the hair follicle and to the epidermis. The sebaceous 

 gland lies between the muscle and the hair follicle. 



The Sweat. Sweat is a clear, colourless fluid containing 99 per cent, 

 of water ; sodium chloride is the most abundant solid constituent, and 

 traces of proteins and of urea may also be present. 



The secretion of sweat is under the control of the central nervous 

 system, the nerves to the sweat glands belonging entirely to the 

 sympathetic system. Leaving the spinal cord by the anterior roots, 

 they pass to the ganglia of the lateral sympathetic chain, where they 

 have their cell station ; from these ganglia non-medullated fibres enter 

 the grey rami, and run with the spinal nerves to their peripheral dis- 

 tribution. Sweating is usually brought about by a rise in the body 

 temperature, and it generally begins as soon as the temperature of 

 the body rises from J to 1 C. above the normal. In this case the 

 effective stimulus is the raised temperature of the blood passing 

 through the brain ; and sweating may be produced by warming the 

 blood passing through the carotid artery to the brain, even though the 

 temperature of the rest of the body remains unchanged. 



Sweating may also be produced reflexly by the local application of 

 heat to the skin, so that one arm, if warmed, may sweat, and not the 

 rest of the body. It is not necessarily associated with increased vascu- 

 larity of the skin, and may occur, when the sympathetic fibres are 

 stimulated, even in an amputated and therefore bloodless limb. 



Hence the skin not only protects the delicate underlying structures 

 and serves as a sense organ, but by means of the secretion of sweat 

 plays an important part in effecting the loss of heat from the body. 

 The loss of heat from the skin is brought about by radiation, con- 

 vection, and evaporation. Heat passes by convection to the air, or to 

 articles of clothing in contact with the body ; it is also lost from the 

 exposed surfaces of the body by radiation to objects at a distance. The 

 loss thus taking place is greater when the blood-vessels of the skin are 

 dilated and the skin is flushed than when the vessels are constricted. 



More important than either of these is the loss of heat by the 

 evaporation of sweat, which is continually being formed on the surface 

 of the skin. When the amount of sweat is small it evaporates so 

 quickly as to be unnoticed, the process being called insensible per- 

 spiration. When the amount formed is increased, or its immediate 

 evaporation is prevented, it becomes visible on the surface of the 

 skin as sensible perspiration. 



