GENERAL CONSIDERATIONS. 525 



were well digested, and the animal throve on a diet of bread and 

 milk. 



The salivary glands, then, in the domestic dog, appear to be rather 

 a convenience than a necessity, and there is no evidence that they have 

 any " internal secretion " ; carbonic acid passes from the gland-cells to 

 the blood, but there is no indication that any other substance does so. 



Saliva injected into the blood is much less harmful than might be 

 expected. Bernard, 1 indeed, injected considerable quantities into a vein 

 of a dog to which no anaesthetics had been given, and did not observe a 

 result of any kind. Extracts of the salivary glands injected into the 

 blood cause a temporary fall of blood pressure, 2 but so many substances 

 in solution do this that the action cannot be regarded as specific. 



GENERAL CONSIDERATIONS ; THEORIES AS TO THE MODE OF ACTION 

 OF SECRETORY NERVES. S 



The facts which show that secretory nerve-fibres exist in the cranial 

 nerves are so well known, that it is not necessary to consider them in 

 detail. It is sufficient here to recall the fundamental facts, that secre- 

 tion may in each salivary gland take place at a pressure higher than 

 that of the blood supplied to the gland, and that nerve-fibres end in 

 connection with the gland-cells. 4 . 



In the case of the sympathetic, the comparatively slight amount 

 and the transitory nature of the secretion, render the question less clear. 

 It was in fact suggested, 5 early in the history of sympathetic saliva, 

 that the cervical sympathetic nerve causes a secretion solely in con- 

 sequence of the pressure exercised on the gland-cells by the contraction 

 of the blood vessels, brought about by stimulation of the nerve. Such a 

 view offers a plausible explanation of many of the facts relating to the 

 secretory action of the sympathetic, such as the normal small quantity 

 of the secretion in the dog, the increased quantity after the cranial nerve 

 has been stimulated, the rapidity with which the maximum rate of the 

 " augmented " saliva is attained, the normal absence of reflex secretion 

 by way of the sympathetic when sapid substances are placed on the 

 tongue, and the absence of effect of atropine and pilocarpine upon the 

 secretory function of the sympathetic. 



But a closer inquiry shows, nevertheless, that this view is untenable. 

 On the general theory it may be noted, that the constriction of the small 

 arteries of the gland in all probability decreases the pressure on the 

 gland-cells instead of increasing it. On the experimental side, we may 

 mention three points. 



1. The constriction of the blood vessels has at times no relation to 



1 "Le9ons de physiol. expe*r.," 1856, p. 141. 



2 Schafer and Oliver, Journ. Physiol., Cambridge and London, 1895, vol. xviii. p. 277. 



3 For a general historical account of the views which have been held with regard to 

 secretion, I may refer the reader to Prof. Gamgee's Address to the Biological Section of the 

 British Association in 1882, and to Prof. Heidenhain's Introductory Account in Hermann's 

 "Handbuch," 1880, Bd. v. Th. 1, S. 1-13. 



4 Cf. Fusari et. Panasci, Arch. ital. de biol., Turin, 1891, tome xiv. ; G. Eetzius, 

 Biol. Untcrsuch., Stockholm, 1892, N. F., Bde. iii., iv. ; Korolkow, Anat. Anz., 

 Jena, 1892, Bd. vii. S. 580; A. Dogiel, Arch. f. mikr., Anat. Bonn, 1893, Bd. xlii. ; 

 Berkeley, Johns Hopkins Hosp. fiep., Baltimore, 1894, vol. v. ; C. Arnstein, Anat. Anz., 

 Jena, 1895, Bd. x. S. 410; G. C. Huber, Journ. Exper. Med., Baltimore, 1896, vol. i. 

 p. 281. 



5 Griinhagen. Ztschr. f. rat. Med., 1868, Bd. xxxiii. S, 258. 



