296 TEXT-BOOK OF PHYSIOLOGY. 



reflected to it through afferent or sensory nerves. The latter suppo- 

 sition is supported by the results of experimentation, though both 

 factors are undoubtedly of importance. 



Stimulation of the sensory nerves in almost any region of the body 

 is followed by a slowing of the heart's action. Thus stimulation of 

 the posterior roots of the spinal nerves, the trunks of the cranial 

 sensory nerves, the splanchnic nerves, the pulmonary branches of the 

 vagus, etc., give rise to a more or less pronounced inhibition. As a 

 rule, stimulation of the peripheral terminations of these nerves is 

 more effective than stimulation of their trunks, hence an explanation 

 is at hand for the cardiac inhibition which results from sudden dis- 

 tention of the stomach, intestines, or lungs. 



The Cardio-accelerator Center. The exact location of this 

 center is not as yet determined. It is probably in the medulla ob- 

 longata. The experiments of Hunt would indicate that Inis "center is 

 also in a condition of tonic activity, increasing both the rate and force 

 of the heart's action; that it antagonizes the action of the inhibitory 

 center and is in time antagonized by it; that the normal rate of the 

 heart from moment to moment is the resultant of the action of these 

 two opposing forces. These experiments also support the view that 

 the acceleration of the heart observed during stimulation of certain 

 afferent nerves is not due to a. reflex stimulation of the accelerator 

 center, but to an inhibition of the cardio-inhibitor center. It must 

 therefore be assumed that the afferent nerves contain two sets of 

 nerve-fibers, one of which inhibits, the other excites, the cardio- 

 inhibitor center. 



The Depressor Nerve. The vagus trunk also contains afferent 

 fibers stimulation of which not only brings about a reflex inhibition 

 of the heart, but also a dilatation of the peripheral arteries and a fall 

 of blood-pressure through an influence exerted on the vaso-motor 

 centers. To this nerve the term depressor has been given. A con- 

 sideration of the physiologic action of this nerve will be found in the 

 section devoted to the nerve mechanisms concerned in the mainte- 

 nance of the blood-pressure. 



THE VASCULAR APPARATUS: ITS STRUCTURE AND FUNCTIONS. 



The systemic vascular apparatus consists of a closed system of 

 vessels extending from the left ventricle to the right auricle, and 

 includes the arteries, capillaries, and veins. Though serving as a 

 whole to transmit blood from the one side of the heart to the other, 

 each one of these three divisions has separate but related functions, 

 which are dependent partly on differences in structure and physio- 

 logic properties, and partly on their relation to the heart and its 

 physiologic activities. 



