270 THE HUMAN BODY. 



the trypsin. To decide between the relative importance of 

 these possible agencies we- must pass to the consideration of 

 other glands; since the question can only be decided by 

 experiment upon the lower animals, and the position of 

 the pancreas and the difficulty of getting at its nerves with- 

 out such severe operations as upset the physiological condi- 

 tion of the animal, furnish obstacles to its study which 

 have not yet been overcome. 



In certain other glands, however, we find conclusive evi- 

 dence of a direct action of nerve-fibres upon the secreting 

 elements. If the sciatic nerve of a cat be stimulated elec- 

 trically the balls of its feet will sweat. Under ordinary 

 circumstances they become at the same time red and full 

 of blood; but that this congestion is a factor of subsidiary 

 importance as regards secretion is proved by the facts that 

 stimulation of the nerve is still able to excite the gland- 

 cells and cause sweating in a limb which has been ampu- 

 tated ten or fifteen minutes (and in which therefore no cir- 

 culatory changes can occur) and also by the cold sweats, 

 with a pallid skin, of phthisis and the death agony. It is, 

 however, with reference to the submaxillary and parotid 

 salivary glands that our information is most precise. 



When the mouth is empty and the jaws at rest the sali- 

 vary secretion is comparatively small: but a sapid substance 

 placed on the tongue will cause a copious flow. The phe- 

 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 

 shortening of the muscular fibres; the other is the dilata- 

 tion of the muscular arteries; every muscular nerve con- 

 tains two sets of fibres, one motor and one vaso-dilator, 

 and normally both act together. In this case, however, 



