INNERVATION OF THE BLOOD-VESSELS OF DIFFERENT ORGANS 419 



motor nerve with currents of medium strength and frequency gives 

 rise to a vasoconstriction in the part innervated by it. This result 

 may also be obtained by stimulation of the intact nerve, and naturally, 

 if a certain nerve is composed solely of dilator fibers, its excitation 

 must be followed by a dilatation. As an example of this kind might be 

 mentioned the chorda tympani which, as has been stated above, 

 consists of dilator fibers for the submaxillary and sublingual glands. 



As far as the result of these constrictor reactions is concerned, it 

 must be evident that the diminution in the caliber of the arterial 

 terminals must reduce the arterial throughflow. This change is asso- 

 ciated with an increase in the arterial pressure and a decrease in the 

 capillary and venous pressures. Conversely, a vasodilatation must 

 favor a greater escape of blood into the capillaries and occasion a fall 

 in the arterial and a rise in the capillary and venous pressures. 



It has previously been emphasized that the vasomotor mechanism 

 is the chief factor concerned in the production of the peripheral re- 

 sistance, and that the latter in turn plays a most important part in the 

 production of blood pressure. The other three factors are the energy 

 of the heart, the total quantity of the blood, and the elasticity of the 

 blood-vessels. Consequently, the blood pressure must be entirely 

 dependent upon the proper interaction of these four values. Thus, it 

 will be seen that the effects of a vasoconstriction may be greatly les- 

 sened by a reduction in the cardiac output, while a vasodilatation may 

 be quite offset by an augmentation of the action of the heart. This 

 compensatory phenomenon is indeed a very common one, because a 

 high blood pressure, resulting in the course of a general vasoconstric- 

 tion, is usually neutralized by a reduction in the cardiac output. But, 

 it may also happen that the other factors act in perfect unison with the 

 vasomotor mechanism and thus occasion an exaggeration of the vaso- 

 motor effect. For example, if a general vasoconstriction occurs syn- 

 chronously with a high cardiac rate, a rise in blood pressure must 

 result which must greatly exceed the rise produced by the vasocon- 

 striction alone. 



Nothing further need be said regarding the pressor and depressor 

 reactions. Inasmuch as these effects are brought about reflexly 

 by impulses generated in different parts of the body, the vasomotor 

 center must be activated first before these impulses can be transferred 

 upon the efferent channels. One or the other of these effects may be 

 elicited either by stimulating the afferent nerve while intact, or by 

 dividing it and using its central end for the stimulation. Obviously, 

 if the distal end of a nerve of this kind is subjected to the excitation, 

 the impulses here generated cannot reach the center at all and hence, 

 no pressor or depressor effect can be evoked. As a typical example of 

 a depressor nerve might be mentioned the depressor cordis, the stimu- 

 lation of which produces a general reflex vasodilatation and a most 

 decided fall in blood pressure. Similar results may be obtained by 

 the excitation of the splanchnic nerve, and especially if currents of 



