580 THE SKIN AND ITS APPENDAGES [CH. XXXVIII. 



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 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 

 to the abdominal sympathetic and 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 (Adam- 

 kiewicz). 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 sweat- 

 ing; thus unilateral perspiration is seen in some cases of hemi- 

 plegia; degeneration of the anterior nerve-cells of the cord may 

 cause stoppage of the secretion. It is sometimes increased in 

 paralysed limbs. 



The changes that occur in the secreting cells have been investi- 

 gated by Eenaut in the horse. When charged they are clear 

 and swollen, the nucleus being situated near their attached ends; 



