178 BIOLOGICAL LECTURES. 



Like the latter, the sweat glands are provided with a muscu- 

 lature between the cells and the basement membrane. There 

 is no evidence of the existence of a sphincter. The intermit - 

 tence of secretion of the sweat glands is produced in two ways. 

 The easiest perceived is that produced by the vascular system. 

 Dilate the blood vessels of the skin by any means, and sweat 

 secretion, under normal conditions, always follows. It makes 

 no difference how the dilation is brought about ; we may pro- 

 duce it by cutting off the circulation from an area entirely for 

 fifteen to twenty minutes. On readmitting the blood, vaso- 

 dilation ensues, and profuse sweating. Cut the cervical sym- 

 pathetic nerve in the neck of the horse, and vaso-dilation, 

 accompanied by heavy perspiration, results. Here, as in all 

 other vertebrate glands, then, other factors being equal and the 

 gland in a normal state, vaso-dilation invariably causes secre- 

 tion. It causes a discharge from the duct as well as from the 

 cells, because there is no bladder and no sphincter. 



There is another way, however, in which sweat secretion is 

 rendered intermittent. This is by the actions of the muscu- 

 lature surrounding the gland. This drives out the fluid in the 

 gland and its duct. This is the secretion which takes place on 

 stimulation of the sciatic, which can go on after death, in the 

 absence of blood supply or during vaso-constriction. If the 

 sciatic nerve of the cat be stimulated, and during stimulation 

 the skin plunged into some powerful and rapid fixative, the 

 muscular sheath is found in strong contraction. This secretion 

 generally coexists with vaso-constriction, since the same agen- 

 cies which cause contraction of. the sheath of the gland cause 

 also contraction of the arterioles. Nerves which act on this 

 sheath will be secretory nerves. Such sweat secretions are 

 caused also by constricting drugs, such as strychnine, picro- 

 toxine, physostigmine, and so forth. The secretion of sweat is 

 controlled, then, in the same two ways as are nearly all other 

 vertebrate intermittent secretions by the action of the nerves 

 on the muscular sheath and by the action of nerves on the 

 blood vessels. 



Another interesting skin secretion is that of the mammary 

 glands. Here we have a constant secretion by the cells, at 



