33 o TEXT-BOOK OF PHYSIOLOGY. 



in the trunk of the facial nerve, and reach the gland by way of the 

 chorda tympani branch. Their cell station is in the ganglion near 

 the hilum of the gland. The vaso-dilatator nerves for the blood- 

 vessels of the corpora cavernosa of the penis, the nervi erigentes, 

 have their origin in the sacral region of the spinal cord; and emerge 

 in the roots of the second and third sacral nerves. Their cell station 

 is in the ganglion near the organ. 



The existence, course, distribution, and functions of the vaso- 

 dilatator fibers have been determined by the same methods employed 

 in the investigation of the vaso-constrictors. Thus division and 

 stimulation of the peripheral end of the chorda tympani nerve are 

 at once followed by an active dilatation of the blood-vessels of the 

 submaxillary gland. The inflow of blood is so great that the gland 

 becomes bright red in color. Its tissues being unable to appropriate 

 all the oxygen, the blood emerges in the veins almost arterial in char- 

 acter. Stimulation of the peripheral ends of the divided nervi 

 erigentes is followed by similar effects in the blood-vessels of the 

 corpora cavernosa. Slow stimulation, once per second, of the periph- 

 eral end of a divided sciatic nerve is followed by dilatation of the 

 blood-vessels of the leg. 



From these and many other facts of a similar character it is highly 

 probable that the blood-vessels of each organ are under the control 

 of two antagonistic classes of nerve-fibers, one augmenting the 

 degree of their contraction, the vaso-constrictors, the other diminish- 

 ing it through inhibition, the vaso-inhibitors. Through the coopera- 

 tive antagonism of these two classes of nerves the caliber of the blood- 

 vessels is accurately adapted to the needs of each organ for blood, 

 both during rest and during activity. It is also to the alternate 

 activity of these nerves that the variations occurring from time to 

 time in the volume of organs are to be attributed. 



The vaso-constrictors and the vaso- dilatators differ somewhat in 

 their physiologic properties, as shown by the results of experiment. 

 Thus, when a mixed nerve, i. e., one containing both classes of fibers, 

 e. g., the sciatic, is stimulated with frequently repeated induced 

 currents, the constrictor effect is the more pronounced, the dilatator 

 effect being wanting or prevented; when stimulated with slowly 

 repeated induced currents, the dilatator effect is the more pronounced. 

 These different effects are strikingly shown in Fig. 156, A and B. 



In the experiment of which these tracings are the result the leg 

 of a cat was enclosed in a plethysmograph and the variations in volume 

 due to dilatation or contraction of the vessels, following stimulation 

 of the sciatic nerve, were recorded by means of a tambour and lever 

 on a slowly revolving cylinder. In A the fall of the curve indicates 

 a diminution of volume, from contraction of blood-vessels following 

 a rate of stimulation of the sciatic nerve of 16 per second for fifteen 



