CHAPTER XX. 



THE CIRCULATION (Cont'd). 



The Influence of the Nervous System on the Circulation. 



Up to the present time we have considered the circulatory 

 system as a purely automatic and mechanical apparatus for 

 carrying of blood to all parts of the body. It is necessary that 

 this apparatus vary in its activity, not only according to the 

 needs of the body as a whole, but also according to the needs of 

 the various parts of the body. It would be poor economy for the 

 heart to maintain through all parts of the body at all times a 

 stream of blood which would be large enough for all emergencies. 

 There must be some way of controlling the blood flow according 

 to the needs of the body. This function is served primarily by 

 the central nervous system, which is connected by means of 

 nerves with the musculature of the heart and the blood vessels, 

 and secondarily by secretions from the so-called ductless glands, 

 the best known of which are the adrenal glands (see p. 129). 



The Nervous Control of the Heart. 



The Cardiac Nerves. The heart is supplied with both sen- 

 sory and motor nerves. Sensory nerves carry stimuli from the 

 peripheral regions to the brain and are known as afferent nerves. 

 Motor nerves, on the other hand, carry stimuli from the brain 

 to the muscles or glands, and are known as efferent nerves. The 

 efferent nerves of the heart are found in fibers coming from the 

 spinal cord by way of the sympathetic system, and by the vagi 

 or the tenth pair of cranial nerves (see p. 265). It must be 

 clearly understood that the nerves merely regulate the heart 

 beat, but have nothing to do with its occurrence. In other words, 

 the heart continues to beat after all the nerves have been severed. 



The Accelerator Nerves. To understand how the fibers 

 reach the heart, the reader is referred to the general description 



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