THE DYSPNCEIC SECRETION. 493 



THE DYSPNCEIC SECRETION. 



At a certain stage of dyspnoea the saliva Hows with considerable 

 rapidity from all the salivary glands. The time at which it begins and 

 its amount are dependent upon the degree of anaesthesia. In anaesthesia, 

 the secretion does not usuajly begin until the stage of expiratory convul- 

 sions. With a large excess of anaesthetics, the animal may be killed by 

 asphyxia without any secretion occurring, or with only a trifling amount. 

 When a copious secretion occurs it is due to impulses passing down the 

 cerebral nerve fibres, but some secretion may be obtained after section 

 of these nerve fibres. In such case there is also contraction of the 

 glandular arteries. Whether dyspnoea is capable of producing a secre- 

 tion after section of the cerebral nerve and excision of the superior 

 cervical ganglion, has not been sufficiently investigated. 



STIMULATION OF THE CRANIAL NERVE SUPPLYING A SALIVARY 



GLAND. 



On some general features of the secretion. A flow of saliva can 

 be obtained from any of the salivary glands by electrical, mechanical, or 

 chemical stimulation of the cranial secretory nerves. It need hardly be 

 said that the interrupted current is the most effective form of stimulus. 

 A very weak interrupted current, which cannot be felt on the tongue, is 

 sufficient to cause a secretion. Within certain limits the rate of flow of 

 the saliva increases with the strength of the stimuli, but strong currents 

 rapidly injure the nerve at the point of stimulation. Even with 

 moderate currents a very slight shifting of the electrodes on the nerve 

 usually causes a marked increase in the rate of secretion, a fact which it 

 is important to bear in mind in collecting for analysis different samples 

 of saliva, secreted under different conditions. 



The flow of saliva with a moderate strength of current is very rapid ; 

 thus the submaxillary gland in the dog may secrete in five minutes an 

 amount of saliva weighing as much as the whole gland. 



The nerve can be stimulated electrically for half an hour to an hour, 

 and probably with proper precautions very much longer, without the 

 flow of saliva ceasing. Pilocarpine in successive doses (cf. p. 513) will 

 cause a secretion for, so far as we know, an indefinite time. 



In protracted electrical stimulation the maximum amount of saliva 

 is obtained by stimulating for short periods, with short intervals of rest ; 

 the stimulation being stopped each time as the secretion becomes slow. 

 In this way in ten to twelve hours about 250 c.c. of saliva can be 

 obtained from the submaxillary gland of the dog, and a half to two- 

 thirds of this amount from the parotid. The rate of flow gradually 

 diminishes during the progress of the experiment. With a given 

 strength of current, the maximum rate of secretion is produced with a 

 rate of interruption of about forty a second. 1 



According to Wedensky, rapid shocks, such as 100 to 250 a second, cause a 

 change in the nerve-endings, so that they soon cease to transmit nervous 

 impulses. His most striking experiment is the following : Two pairs of 

 electrodes are placed on the chorda tympani, shocks of moderate rate are 

 passed through the lower, and of rapid rate through the upper ; the secretion 

 1 Wedensky, Compt. rend. Acad. d. sc., Paris, 1892. 



