THE SECRETORY TISSUES AND ORGANS. 293 



nomenon is closely comparable to the production of a reflex 

 muscular contraction. A stimulus acting upon an irritable 

 tissue excites through it certain afferent nerve-fibres; these 

 excite a nerve-centre, which in turn stimulates efferent fibres; 

 going to a muscle in the one case, to a gland in the other. 

 It will be useful to consider again for a moment what occurs 

 in the case of the muscle, taking account only of the efferent 

 fibres and the parts they act upon. 



When a muscle in the Body is made to contract reflexly, 

 through its nerve, two events occur in it. One is the short- 

 ening of the muscular fibres; the other is the dilatation of 

 the muscular arteries; every muscular nerve contains two 

 sets of fibres, one motor and one vaso-dilator, and normally 

 both act together. In this case, however, it is clear that the 

 activities of both, though correlated, are essentially inde- 

 pendent. The contraction is not due to the greater blood- 

 flow, for not only can an excised muscle entirely deprived of 

 blood be made to contract by stimulating its nerves, but in 

 an animal to which a small dose of curari the arrow-poison 

 of certain South American Indians has been given, stimu- 

 lation of the nerve will cause the vascular dilatation but no 

 muscular contraction: the curari paralyzing the motor fibres, 

 but, unless in large doses, leaving the vaso-dilators intact. 

 The muscular fibres themselves are unacted upon by the poi- 

 son, as is proved by their ready contraction when directly 

 stimulated by an electric shock. 



Now let us return to the salivary glands and see how 

 far the facts are comparable. The main nerve of the sub- 

 maxillary gland is known as the chorda tympani. If it be 

 divided in a narcotized dog, and a tube placed in the gland- 

 duct, no saliva will flow. But on stimulating the peripheral 

 end of the nerve (that end still connected with the gland) 

 an abundant secretion takes place. At the same time there 

 is a great dilatation of the arteries of the organ, much more 

 blood than before flowing through it in a given time: the 

 chorda obviously then contains vaso-dilator fibres. Now in 

 this case it might very well be that the process was different 

 from that in a muscle. It is conceivable that the secretion 

 may be but a filtration due to increased pressure in the gland 

 capillaries, consequent on dilatation of the arteries supplying 

 them. If a greater filtration into the lymph spaces of the 

 gland took place, this liquid might then merely ooze on 



