356 MATHEWS. 



On stimulating the chorda the secretion may be derived from 

 unpoisoned alveoli of which the blood vessels have not hitherto 

 been in dilation. 



The value of Langley's and Heidenhain's observation, that 

 the secretory fibres of the chorda tympani recover, after nico- 

 tine poisoning, before the dilator fibres, is seriously impaired by 

 a defective method of determining whether vaso-dilation did, or 

 did not, occur. If we admit that the rate of flow of blood from 

 the gland's vein is a criterion by which we can determine 

 whether vaso-dilation has or has not occurred their conclusion 

 is justified. But reflection shows that if vaso-dilation be slight 

 the amount of water passing out into the secretion might so re- 

 duce the bulk of blood flowing through the gland as to mask 

 entirely all effects of the increased flow due to vaso-dilation. 

 In fact, the flow of blood from the vein would be a safe cri- 

 terion of dilation, only if there were no escape of liquid through 

 the capillary wall, a condition which manifestly does not here 

 exist. Langley's and Heidenhain's conclusion that the secre- 

 tory function recovers before the dilator is, hence, unjustified. 

 The same criticism applies, also, to Heidenhain's observation 

 that after the chorda tympani has been cut and allowed to de- 

 generate for three or four days stimulation still causes an in- 

 crease in the paralytic secretion, but no increase in blood-flow 

 from the vein. 



e. Evidence of the Osmotic Character of the Salivary 

 Secretions which are Accompanied by Vaso-dilation. 



I wish now to summarize briefly those features of secretions, 

 accompanied by vaso-dilation, which indicate that they are of an 

 osmotic character. 



(l) In structure the salivary glands have all the require- 

 ments of an elaborate osmotic mechanism They are, essentially, 

 extraordinarily thin -walled bags, possessing an enormous sur- 

 face, containing a mass of hydroscopic indiffusible substances. 

 The outer surface of this bag is in intimate association with a 

 mesh work of capillaries so coordinated by the nervous system 

 as to permit an almost instantaneous flooding of the gland mem- 



