SECRETIOX. 223 



this case the secretory fibres are still irritable, since stimulation of the chorda 

 gives the usual secretion. 



A still more marked difference between the effect of stimulation of the 

 cerebral and the sympathetic fibres may be observed in the case of the parotid 

 gland in the dog. Stimulation of the cerebral fibres alone in any part of 

 their course (see Fig. 51) gives an abundant thin and watery saliva, poor in 

 solid constituents. Stimulation of the sympathetic fibres alone (provided the 

 cerebral fibres have not been stimulated shortly before (Laugley) and the tym- 

 panic nerve has been cut to prevent a reflex effect) gives usually no perceptible 

 secretion at all. But in this last stimulation a marked effect is produced upon 

 the gland, in spite of the absence of a visible secretion ; this is shown by the 

 fact that subsequent or simultaneous stimulation of the cerebral fibres gives a 

 secretion very unlike that given by the cerebral fibres alone, in that it is very 

 rich indeed in organic constituents. The amount of organic matter in the 

 secretion may be tenfold that of the saliva obtained by stimulation of the 

 cerebral fibres alone. 



Another important and suggestive set of facts with regard to the action of 

 the secretorv nerves is obtained from a study of the differences in composition 

 of the secretion following upon variations in the strength of stimulation of the 

 nerves. 



Relation of the Composition of the Secretion to the Strength of Stimula- 

 tion. — If the stimulus to the chorda is gradually increased in strength, 

 care being taken not to fatigue the gland, the chemical composition of the 

 secretion is found to change with regard to the relative amounts of the 

 water, the salts, and the organic material. The water and the salts increase 

 in amount with the increased strength of stimulus up to a certain maximal 

 limit, which for the salts is about 0.77 per cent. It is important to observe 

 that this effect may be obtained from a perfectly fresh gland as well as 

 from a gland which had previously been secreting actively. With regard 

 to the organic constituents the precise result obtained depends on the con- 

 dition of the gland, li' previous to the stimulation the gland was in a 

 resting condition and unf'atigued, then increased strength of stimulation i- 

 followed at first by a rise in the percentage of organic constituents, and this 

 rise in the beginning is more marked than in the case of the salts. lint 

 with continued stimulation the increase in organic material soon ceases, and 

 finally the amount begins actually to diminish, and may fall to a low point 

 in spite of the stronger stimulation. On the other hand, if the gland in the 

 beginning of the experiment had been previously worked to a considerable 

 extent, then an increase in the stimulating current, while it increases the 

 amount of water and salts, may have either no effect at all upon the organic 

 constituents or cause only a temporary increase, quickly followed by a fall. 

 Similar results may be obtained from stimulation of the cerebral nerves of 

 the parotid gland. The above facts led Heidenhain to believe that the con- 

 ditions determining the secretion of the organic material are different from 



