THE DYSPNQ2IC SECRETION. 493 
The Dyspnceic Secretion. 
At a certain Btage of dyspnoea the saliva flows with considerable 
rapidity from all the salivary glands. The lime at which it begins and 
its amount arc dependent upon the degree of anaesthesia. In anaesthesia, 
the secretion does not usually begin until the stage of expiratory convul- 
sions. With a large e: ■-■ -- of anaesthetics, the animal may be killed by 
asphyxia without any secretion occurring, or with only a trilling 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 dose's (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 Bhort 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- 
thuds 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, Cornet, rend. Acad,, d. sc, Paris, 1892. 
