THE SKIN AND THE SKIN-GLANDS 1165 



carotid arteries on its way to the brain, at a time when the temperature of 

 the blood circulating through the rest of the body, including the paws them- 

 selves, has undergone no alteration. Sweating may also be aroused by 

 asphyxia, and this result is found even in the spinal cat, i.e. after separation 

 of the spinal centres from the medulla. The secretion of sweat resulting 

 from stimulation of the sweat-nerves, although generally associated with 

 increased vascularity of the skin, is not in any way dependent thereon. 

 Thus even in the amputated limb stimulation of the sciatic nerve may cause 

 the appearance of drops of sweat on the pad of the foot. If the sciatic nerve 

 be stimulated in the intact animal, the secretion of sweat which is produced 

 is associated with constriction of the vessels of the skin, due to simul- 

 taneous stimulation of the vaso- constrictor nerves running in the sciatic 

 nerve. 



As Langley has shown, the sweat-nerves run entirely in the sympathetic 

 system. Leaving the cord by the white rami communicantes from the 

 second dorsal to the third or fourth lumbar nerves, they pass into the 

 sympathetic chain. Here the first relay of fibres ends in connection with the 

 cells of the sympathetic ganglia, and a fresh relay of fibres, which are non- 

 medullated, pass from the cells along the grey rami into the various spinal 

 nerves, to be distributed to the whole surface of the skin. The secretion of 

 sweat by the sweat-glands may be roused by the injection of pilocarpine even 

 after division of the sweat-nerves, so that this drug must act peripherally 

 on the glands. The action of pilocarpine, as well as the effects of artificial 

 stimulation of the sweat-nerves, is abolished by the administration of 

 atropine. 



THE GASEOUS EXCHANGES OF THE SKIN. In any animal with a 

 thin moist skin, such as the frog, the absorption of oxygen and the excretion 

 of C0 2 from the skin may be sufficient for the proper aeration of its blood, 

 so that it may continue to live after the extirpation of its lungs. In man 

 there is also a continuous output of C0 2 through the skin, but the amount 

 leaving the body in this way is negligible compared with that which is 

 exhaled through the lungs. The loss of C0 2 by the skin rises with increase 

 of external temperature. Thus at a temperature of 29 to 33 C. the C0 2 

 output by the skin is about 0-35 grm. per hour, i.e. about 84 grm. in the 

 twenty- four hours. When the external temperature rises above 33 C., 

 the C0 2 output increases, so that at 34 it is doubled and at 38-5 it may 

 amount to as much as 1-2 grm. per hour (Schierbeck). It is just at this 

 temperature of 33 C. that a secretion of sweat begins to be noticeable, so it 

 has been suggested that the increased C0 2 output may be due directly to the 

 increased work and metabolism of the sweat-glands during their activity. 



ABSORPTION BY THE SKIN. In order to test the alleged influence of 

 baths containing medicinal substances in solution, many experiments have 

 been made to determine whether absorption is possible by the skin. It 

 may be regarded as established that the uninjured skin is impermeable 

 for watery solutions of salts or other substances. On the other hand, it is 

 possible to produce a certain amount of absorption by the inunction of 



