502 SALIYA [CH. XXXII, 



The loss of water from the arterial blood can be calculated by examin- 

 ing the venous blood, and finding the increase in the concentration 

 of haemoglobin there; it has been experimentally proved that the 

 amount of water lost from the blood is equal to the quantity of saliva 

 formed in a given time. 



We now pass to the second phase of secretion, namely, the altera- 

 tion in the composition of the fluid. The cell is not only a pumping 

 engine for the movement of water, but is also a factory of organic 

 substances which are thrust into the stream. The two most impor- 

 tant of these materials, though neither is of constant occurrence, are 

 mucin, and the enzyme ptyalin. On the other hand, the cell offers 

 an obstruction to the passage of salts ; the saliva is therefore poorer 

 (and the lymph in the lymph space correspondingly richer) in salts 

 than the blood. The accumulation of salts in the lymph is an addi- 

 tional factor in attracting (by osmotic pressure) water out of the 

 blood. Osmosis, however, is only a contributory cause of the flow of 

 lymph; it will not account for secretion of saliva; in fact, if saliva 

 and blood were placed in an osmometer, fluid would pass from the 

 saliva to the blood. 



Secretory Nerves of the Salivary Glands. 



The submaxillary gland has a double nerve-supply. (1) The 

 chorda tympani ; this is a branch of the seventh cranial nerve, and 

 in part of its course is bound up in the same sheath as the lingual 

 nerve, a branch of the fifth. When the lingual nerve crosses 

 Wharton's duct beneath the tongue, the chorda tympani leaves the 

 lingual, and the preganglionic fibres in it for the submaxillary pass 

 into the hilus of that gland, and end by arborising around a scattered 

 collection of ganglion cells concealed within the substance of the 

 gland. This ganglion is known as Langley's ganglion. From the 

 cells of Langley's ganglion post-ganglionic fibres are distributed to 

 the gland-cells and also to the blood-vessels. 



(2) Sympathetic branches are derived from the plexus around 

 the facial artery and accompany the arterial branches which supply 

 the gland. (See fig. 353.) 



The chorda tympani is par excellence the secretory nerve of the 

 gland. When it is stimulated, secretion of saliva and dilatation of 

 the arterioles take place invariably. Stimulation of the sympathetic 

 always produces constriction of these blood-vessels, and a secretion 

 of a small .quantity of thick viscid saliva may also occur, but often 

 the salivary flow is entirely absent. Eecent investigations have 

 shown that the part played by the sympathetic is so inconstant, and 

 the results obtained by different methods of stimulation (e.g., 

 electrical, and the administration of adrenaline) are so different, 



