ii EXTERNAL DIGESTIVE SECRETIONS 73 



the nerve excites a stream of fluid which passes from the blood 

 capillaries to the lymph spaces, and thence to the glandular spaces, 

 the material contained in the gland not being sufficient for such 

 an abundant flow of saliva. 



(&) Between the excitation of the nerve and the appearance 

 of the secretion there is an appreciable period of latent stimula- 

 tion, which may vary between 1-2 seconds (Hering) and 24 seconds 

 (Ludwig). 



The secretion persists for a short time after the close of 

 stimulation. This period, known as the after-effect, increases in 

 proportion with the excitability of the nerve (Ludwig). In fact, 

 if the stimulation lasts only for a short time, so that the nerve 

 is not excessively fatigued, the after-effect lasts longer. These 

 phenomena confirm the dependence of the secretion upon the 

 activity of the secretory nerve. 



(c) Excitation of the cervical sympathetic (or of the 

 sympathetic fibres that accompany the carotid and run to the 

 submaxillary gland) also produces a secretion of saliva, although 

 much more slowly and in smaller quantities. Moreover, while 

 the saliva obtained by stimulation of the cranial nerve is watery 

 and slightly viscid, and shows under the microscope very fe'w 

 salivary corpuscles and granulations, the saliva secreted by excita- 

 tion of the sympathetic is very dense, viscid, ropy from large 

 quantities of mucus, and contains numerous corpuscles and 

 granulations (Eckhard, 01. Bernard). 



(d) The blood-supply of the gland is modified in contrary 

 directions by stimulation of the chorda tympani and of the 

 sympathetic. In the former it is enormously increased by active 

 vascular dilatation, in the latter diminished by active constric- 

 tion (see Vol. I. p. 342 et seq.). It cannot be denied that the 

 marked difference in quantity, density, and viscidity of the 

 saliva obtained on exciting the two kinds of nerves depends at 

 least in part on the varying blood-supply to the gland in the 

 two cases. The difference is much reduced if the gland be filled 

 with blood by a brief stimulation of the chorda tympani before 

 exciting the sympathetic. In this case excitation of the sym- 

 pathetic produces a more copious and less viscid saliva. 



Burton -Opitz (1904), using Hlirthle's haemodrometer, esti- 

 mated the velocity of circulation in dogs in the branches of the 

 external jugular vein, and found it normally very low. By 

 stimulating the intact chorda tympani. it was possible to increase 

 it from two- to six-fold. Stimulation of the sympathetic, on the 

 contrary, determined an almost complete arrest of the circulation. 

 Longer stimulation of the vasoniotors fatigues them, and the 

 circulation then returns to the normal rate, even if the stimular 

 tion be continued. 



(e) The saliva secreted after stimulation of the cranial nerve 



