I 

 THE CIRCULATION OF THE BLOOD. 



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 the local centres, which may be consid- 

 ered to be sufficient under ordinary circumstances to maintain the tone 

 of the vessels. The observations upon the functions of the vaso-motor 

 nerves themselves appear to divide them into four classes: (1) those on 

 division of which dilatation occurs for some time, and which on stimu- 

 lation of their peripheral ends produce constriction; (2) those on division 

 of which momentary dilatation followed by constriction 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 stimula- 

 tion is applied; (4) a class, division of which produces no effect but 

 which, on stimulation, cause according to their function either dilata- 

 tion 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 glands; 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 

 all of these nerves act by inhibiting or augmenting the action of the local 

 nervous mechanism already referred to; and as they are in connection 

 with the central nervous system, it is through them that the medullary 

 and spinal centres are capable of altering 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 

 nervous 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. 



fi. Indirectly, by impressions descending from the cere- 

 brum, e. g., in sudden pallor. 



y. Reflexly, by stimulation of sensory nerves anywhere. 

 (3.) By stimulation of the centres in spinal cord. 



Possibly directly or indirectly, certainly reflexly. 

 (3.) By stimulation of the local centres for each vascular area, 



