CH. XL.] THE SWEAT 609 



THE SWEAT 



Physiology of the Secretion of Sweat. We 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 is 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 con- 

 striction of the vessels to diminished, perspiration. 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 

 thence to the sciatic nerve. They are controlled by a centre in the 

 upper lumbar region of the cord ; those for the upper limbs leave the 

 cord by the sixth, seventh, and eighth anterior thoracic roots, have 

 cell-stations in the ganglion stellatum, and ultimately pass to the 

 ulnar and median nerves; they are controlled by a centre in the 

 cervical enlargement of the cord. The secretory fibres for the head 

 pass in the cervical sympathetic, and in some branches of the fifth 

 cranial nerves. These subsidiary centres are dominated by one in 

 the medulla oblongata (Adamkiewicz). These facts have been 

 obtained by experiments on animals (cat, horse). 



The sweat-centres may be excited directly by venous blood, as in 

 asphyxia ; or by over -heated blood (over 45 C.) ; or by certain drugs 

 (see further) ; or reflexly by stimulation of afferent nerves such as 

 the crural and peroneaL 



Nervous diseases are often accompanied with disordered sweating ; 



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