300 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



to pulsate synchronously with the heart. Evidently, then, stimulation 

 of the chorda tympani nerve increases the blood-supply of this gland, 

 either through an active dilatation of the vessels, or more probably 

 through an inhibition of a local vaso-motor centre. An analogous result 

 will be seen in the case of the depressor nerve, a nerve whose stimulation 

 produces paralysis of the vaso-motor centre and consequent dilation of 

 the blood-vessels. Two results then follow stimulation of the chorda 

 tympani,— an abundant secretion of saliva and a marked hyperemia of 

 the gland. Before, however, the relation between these results are dis- 

 cussed, the influence of the sympathetic nerve on the submaxillary gland 

 must be alluded to. 



As is well known, a constant result of stimulation of a fibre of the 

 sympathetic system is a contraction of the arterioles, and a consequent 

 diminution of the supply of blood in the parts supplied by the nerve. If 

 the filament which leaves the superior cervical ganglion and passes to 

 the submaxillary gland along the carotid is irritated with a weak induction 

 current, there is a momentary flow of saliva, and the character of the 

 secretion so produced differs from that which follows stimulation of the 

 chorda. Sympathetic saliva is very viscid, and can be drawn out in a long 

 thread from the orifice of the cannula. It is of higher specific gravity 

 and richer in organic elements than that which follows stimulation of the 

 chorda. In other words, the chorda saliva contains a maximum quantity 

 of water and a minimum of organic elements, while in sympathetic saliva 

 the proportions are reversed. So, also, the effects of the sympathetic 

 stimulation on the blood-supply of the submaxillary gland differ from 

 those of the chorda tympani. If the sympathetic filament is irritated, the 

 arborescent vessels, especially over the surface of the gland, disappear, 

 and the tissue of the gland becomes pale and the vein of the gland con- 

 tracted and carrying a small quantity of black blood. In fact, therefore, 

 in both respects the function of the sympathetic and chorda tympani 

 nerves are antagonistic; and if each nerve be stimulated alternately at 

 short intervals with the current which applied alone to either nerve 

 would produce its characteristic effect, there is no result. Evidently, 

 then, there is a complete opposition in function in these two nerves. 

 But is the secretion of saliva simply dependent upon the vascular con- 

 dition of the glands? Does the gland act as a sponge, filtering out the 

 saliva from the material within the blood, the quantity being solely 

 dependent upon the quantity of blood in the organ; or is there some 

 special function possessed by the cells of the salivary glands, by which 

 the saliva is separated from the blood without being dependent solely 

 upon the supply of blood ? In other words, what is the mechanism by 

 which the salivary glands separate the salivary secretion from the blood? 

 We know that as the blood passes through the capillaries of the systemic 



