DIGESTION 169 



of blood to the gland produced by stimulating the chorda is not 

 the essential cause of the secretion, is proved by the fact that 

 the pressure of the saliva in the duct of the gland is higher than 

 the blood-pressure within the vessels. Further, if before stimu- 

 lating the chorda some atropine be injected, stimulation of the 

 nerve still produces to the full all the vascular changes, but not 

 a trace of saliva is secreted. Hence, secretion is not due merely 

 to increased blood-pressure. This atropine experiment proves 

 the existence in the chorda of two sets of nerves — viz., secretory 

 and vaso-dilator ; owing to the action of atropine, the secretory 

 nerves are paralysed, while the vaso-dilators are not. And in 

 the sympathetic two sets of nerves can similarly be demon- 

 strated — secretory and vaso-constrictor — though it is most likely 

 that in the majority of animals the secretory fibres in the sym- 

 pathetic are few in number. Pilocarpine is antagonistic to atro- 

 pine, and produces a profuse flow of saliva. 



A peculiar phenomenon is observed in connection with salivary 

 secretion after division of the chorda. Though the gland 

 is cut off from its secretory nerve, yet one or two days after 

 section a secretion appears, and may continue for some weeks 

 until the gland undergoes atrophy. This is known as ' paralytic 

 secretion.' The cause of paralytic secretion is not definitely 

 known. The cut portion of nerve in connection with the gland 

 degenerates ; it has been suggested that the secreting cells are 

 controlled by inhibitory fibres in the nerve, and that the cells, in 

 consequence of a local nervous mechanism in the gland, continue 

 secreting. Very little of a definite nature is known of the para- 

 lytic phenomenon, and it is curious to observe that it is not 

 limited to the gland of which the nerve has been divided, but 

 affects both sides. Langley, who has described this, refers to 

 the continuous secretion from the unoperated gland as anti- 

 paralytic or antilytic. 



Heidenhain's view of the action of secretory nerves is that a 

 gland is supplied with a trophic or nutritive nerve which excites 

 the formation of the organic constituents of the secretion, and 

 a secretory nerve which controls the secretion of water and 

 inorganic salts. The cranial nerves are chiefly secretory, while 

 the sympathetic are trophic, or building up ; hence stimulation 

 of the chorda yields the water and salts of the saliva, while 

 stimulation of the sympathetic produces the organic substances 

 and ferment. 



Trophic fibres are supposed to effect a breaking-down of the 

 complex living substance of the gland and conversion into simpler 

 bodies, and that chemical changes of importance are occurring 

 is undoubted from the large amount of C0 2 found in saliva, which 

 indicates active oxidation. Saliva contains more C0 2 than any 



