174 BIOLOGICAL LECTURES. 



around the duct, holding the secretion in so that it accumulates 

 under high pressure. By the dilation of the sphincter the 

 secretion pushed from behind rushes forth. Or, second, the 

 gland or its duct is provided with a muscular sheath, and the 

 secretion is driven out by the contraction of the sheath. In 

 both these cases the secretion from the cell is constant ; it is 

 only the discharge from the ducts which is intermittent. In 

 the second method mentioned above, two modifications exist ; 

 the muscular duct may be dilated into a bladder, or the mus- 

 cular sheath may be around the gland. 



Among the secretions which are rendered intermittent by 

 the dilation of a sphincter may be mentioned the secretion of 

 silk in the Lepidoptera. In this case three chitinous jaws 

 press the thread, holding it firm ; the thread accumulates under 

 pressure in the rear. When the jaws are drawn apart by 

 muscle the thread rushes forth. Other instances of inter- 

 mittence produced by sphincters are the segmental organs 

 of leeches, in which the vesicle is emptied by pressure, after 

 dilation of a sphincter ; the odoriferous glands of the Forfi- 

 culidae ; the poison glands of insects. 



The second method in which a constant secretion accumulates 

 in a bladder with muscular walls provided with a sphincter is 

 also common among invertebrates. Examples of such secretion 

 are exhibited by the green glands of lobsters, the salivary glands 

 of certain mollusks, the salivary glands of bees. Among the 

 cases in which the gland itself is supplied by a muscular tunic, 

 by the contractions of which it is emptied, may be mentioned 

 the salivary glands of cephalopods, the poison glands of spiders, 

 the pericardial glands of lamellibranchs, the livers of all Crusta- 

 cea and mollusks. It may be pointed out that the nerves going 

 to the musculature in all these cases will control the discharge 

 from the gland and become in this sense secretory nerves. 



In the vertebrates the cases in which muscle controls the 

 intermittence are also numerous and unmistakable. Let us 

 consider first the simplest and most obvious case the kidney. 

 The secretion of the kidney cells, the real secretion of the kid- 

 ney, is continuous. It is, however, directly under the control 

 of the vascular system. The secretion from the duct of the 



