THE CIRCULATION 297 



faculty. This influence probably comes primarily from the cortex of 

 the frontal lobes. On the other hand, many emotions and mental 

 excitement in general hasten the heart, these impulses coming perhaps 

 from the optic thalami, the possible centers of emotional expression. 

 Depressive or asthenic emotions (for example, terror) may inhibit the 

 heart at first even to complete and sometimes permanent stand-still, 

 as one sees too often in practical jokes with "ghosts," etc., played on 

 "nervous" people. (See Expts. 85, 86, and 87, in the Appendix.) 



THE AFFERENT NERVE OF THE HEART in man runs its fibers in the 

 vagus trunk, but in the rabbit it is a separate nerve. It is distributed 

 over the ventricles, and centrally seems to be in close relation with the 

 vasomotor centers located probably in the medulla. Stimulation of the 

 peripheral stump after the oierve has been cut gives no effect appreciable, 

 but stimulation of the central stump causes often a halving of the blood- 

 pressure (due probably to general vaso-dilatation) and a diminution of 

 the pulse-rate. Both of these effects are in the direction of reducing 

 the labor of the heart, and this is apparently one of the most important 

 functions of this nerve. The depressor seems to be in relation with the 

 sensory centers of the brain, for when the central stump is stimulated 

 in an animal not unconscious there are sometimes signs of pain. These 

 afferent fibers of the vagus received their name "depressor" from the 

 lowering of blood-pressure which its stimulation produces, but, as we 

 shall soon see, the same effect may come in other ways which affect the 

 vaso-motor (vaso-dilator ?) centers. 



CARDIAC CENTERS. As has been hinted already, it is probable that 

 the medulla oblongata is the site of knots of fibers and of nerve-cells 

 which jointly or severally control the impulses passing to the heart. 

 They also connect these impulses and those coming from the heart with 

 other "centers," especially with those which regulate the tonus of the 

 blood-vessels. The former conception of a nerve-center has about gone 

 by, leaving us in ignorance of how the fibrils of these various nerves are 

 connected in the medulla. We may be confident, however, that each 

 nerve does not have a localized and independent area of nerve-tissue 

 from which messages are sent out as from one desk in a large telegraph 

 office, quite independently of all the others. The obvious organic 

 complexity makes unlikely any such simplicity in the control of the heart. 

 One must think, then, of a center as an ordered system of fibers or of 

 fibrils dominated in some way or ways by nerve-cells, trophically, and 

 possibly but not probably as centers of force. 



The Functions of the Blood- and Lymph-vessels. These may well 

 be described under the headings of the four sorts of vessels : arteries, capil- 

 laries, lymphatics, and veins. Each takes a somewhat different part in 

 the circulation, although all of course are primarily the distributing tubes 

 of the one circulation. Their differences depend on the respective 

 uses of the vessels: the arteries distribute the blood in a complicated way; 

 the capillaries are its somewhat loosely defined channels through the 

 tissues; the lymphatics return the osmosed plasma to the blood-vessels 



