CH. xxxvnt.] THE SWEAT. 577 



with it ; thus in some cases infants who will not take cod-liver 

 oil by the mouth, can yet be dosed with it by rubbing it into the 

 skin. Many ointments also are absorbed, and thus general effects 

 produced by local inunction. 



Secretion. The secretions of the skin are two in number. 

 The sebum is the natural lubricant of the hairs. The sweat is an 

 excretion. The secretion of sweat is an important function of 

 the skin, and we will therefore discuss it at greater length. 



THE SWEAT. 



Physiology of the Secretion of Sweat. W'e have seen that 

 the sweat-glands are most abundant in man on the palms and 

 soles, and here the greatest amount of perspiration occurs. 

 Different animals vary a good deal in the amount of sweat they 

 secrete, and in the place where the secretion is most abundant. 

 Thus the ox perspires less than the horse and sheep ; perspiration 

 is absent from rats, rabbits, and goats ; pigs perspire mostly on 

 the snout ; dogs and cats on the pads of the feet. 



As long as the secretion is small in amount, it is evaporated 

 from the surface at once ; this is called insensible perspiration. 

 As soon as the secretion is increased or evaporation prevented, 

 drops appear on the surface of the skin. This is known as 

 sensible perspiration. The relation of these two varies with the 

 temperature of the air, the drier and hotter the air, the greater 

 being the proportion of insensible to sensible perspiration. In 

 round numbers the total amount of sweat secreted by a man is 

 two pounds in the twenty-four hours. 



The amount of secretion is influenced by the vaso-motor 

 nerves ; an increase in the size of the skin-vessels leads to 

 increased, a constriction of the vessels to diminished, perspira- 

 tion. There are also special secretory fibres, stimulation of 

 which causes a secretion even when the circulation is suspended, 

 as in a recently amputated limb. These fibres are paralysed 

 by atropine. They are contained in the same nerve-trunks as 

 the vaso-motor nerves, as are also the nerve-fibres which supply 

 the plain muscular fibres of the sweat-glands which act during the 

 expulsion of the secretion. The secretory nerves for the lower 

 limbs issue from the spinal cord by the last two or three dorsal 

 and first two or four lumbar nerves (in the cat) ; they have cell 

 stations in the lower ganglia of the lateral chain, and pass to the 

 abdominal sympathetic and thence to the sciatic nerve. They 

 are controlled by a centre in the upper lumbar region of the cord ; 



K.I', p P 



