156 HAND-BOOK OF PHYSIOLOGY. 



local stimulation, e.g., the application of heat or cold, will cause dilatation 

 or constriction. From this it is probable that there exists a local 

 mechanism distinct for each vascular area, and that the effect produced 

 by the central nervous system acts through it much in the same way as the 

 cardio-inhibitory centre in the medulla acts upon the heart through the 

 ganglia contained within its muscular substance. 



Central impulses may inhibit or increase the action of these local 

 centres, which may be considered to be sufficient under ordinary circum- 

 stances to maintain the local tone of the vessels. The observations upon 

 the functions of the vaso-motor nerves appear to divide them into four 

 classes: (1) those on division of which dilatation occurs for some time, 

 and which on stimulation of their peripheral end produce constriction; 

 (2) those on division of which momentary dilatation followed by constric- 

 tion occurs, with dilatation on stimulation; (3) those on division of which 

 dilatation is caused, which lasts for a limited time, with constriction if 

 stimulated at once, but dilatation if some time is allowed to elapse before 

 the stimulation is applied; (4) a class, division of which produces no 

 effect but which, on stimulation, cause according to their function either 

 dilatation or constriction. A good example of this fourth class is afforded 

 by the nerves supplying the submaxillary gland, viz., the chorda tympani 

 and the sympathetic. When either of these nerves is simply divided, 

 no change takes place in the vessels of the gland; but on stimulating the 

 chorda tympani the vessels dilate, and, on the other hand, when the 

 sympathetic is stimulated the vessels contract. The nerves acting like 

 the chorda tympani in this case are called vaso-dilators, and those like 

 the sympathetic vaso-constrictors. The third class, which produce at one 

 time dilatation, at another time constriction, are believed to contain both 

 kinds of vaso-motor nerve-fibres, or to act as dilators or contractors 

 according to the condition of the local apparatus. It is probable that 

 these nerves act by inhibiting or augmenting the action of the local nerv- 

 ous mechanism already referred to; and as they are in connection with 

 the central nervous system, it is through this arrangement that that sys- 

 tem is capable of influencing or of maintaining the normal local tone. 



It may also be supposed that the local nerve-centres themselves may 

 be directly affected by the condition of blood nourishing them. 



The following table may serve as a summary of the effect of the nerv- 

 ous system upon the arteries and so upon the blood-pressure: 



A. An increase of the blood-pressure may be produced: 



(1.) By stimulation of the vaso-motor centre in medulla, either 

 a. Directly, as by carbonated or deoxygenated blood. 

 /3. Indirectly, by impressions descending from the cerebrum, 



e.g., in sudden pallor. 

 y. Reflexly, by stimulation of sensory nerves anywhere. 



