THE PHYSIOLOGY OF SECRETION. 179 



least nearly constant in some cases. So far as it is intermit- 

 tent, it is controlled by the vascular system. After parturition 

 the glands are gorged with blood, and secretion from the cells 

 begins. The intermittence of the discharge of the secretion is 

 brought about by muscles, just as it is in the other secretions 

 examined. In this case it is largely the action of a sphincter, 

 though doubtless the muscle of the ducts and about the alveoli 

 comes into play. The cells secrete constantly against the 

 sphincter, until, like the frog-skin glands or the poison glands 

 of spiders, a high pressure is generated. The gland is put on 

 the stretch. This pressure may even become so great that the 

 sphincter can no longer restrain the milk which issues from 

 the duct. The intermittence is caused by the dilation of the 

 sphincters and the contraction of the muscle around the 

 alveoli. In the mammary glands, also, evidence of the exist- 

 ence of nerves acting on the gland cells is lacking. 



We come now to the most interesting glands of all — the 

 vertebrate salivary glands. They are the most interesting, be- 

 cause they are the most complex, the most obscure, and possi- 

 bly on account of their obscurity they have furnished nearly 

 the whole of the evidence for the existence of special secre- 

 tory nerves acting on the gland cells themselves. How are 

 these glands rendered intermittent } In the light of the com- 

 parative study we have been through I have no hesitation in 

 saying that they are rendered intermittent in just the same 

 two ways as are all the other glands, i.e., by the action of the 

 vasomotors and by the action of muscle. It is another case of 

 the gland being inside the bladder. 



That the submaxillary gland is regulated in its secretion by 

 the blood flow, not the slightest doubt can exist. The resting 

 gland, like the pancreas, is white ; the active gland, red ; the 

 chief secretory nerve is a dilator; vaso-dilation, as long as the 

 gland is unpoisoned, always causes secretion. Thus, if the blood 

 be cut off for fifteen or twenty minutes and then readmitted, 

 vaso-dilation and secretion ensue. If one extirpate the sub- 

 lingual ganglion leading to dilation in that gland, spontaneous 

 secretion begins and continues. These glands are rendered 

 intermittent, then, by the vascular changes going on in them. 



