252 THE HUMAN BODY. 



centre, where they are reflected as efferent impulses down 

 the pneumogastric trunks to the heart. In man and other 

 mammals similar arrangements exist, the afferent fibres pass- 

 ing from the alimentary canal through the solar plexus (p. 172} 

 which lies behind the stomach. It is by exciting them and 

 so reflexly stopping the heart, that men are sometimes killed 

 by a severe blow on the abdomen or even occasionally by a 

 large draught of very cold water, the sudden cold acting as 

 a thermal stimulus, through tlie walls of the stomach, on the 

 nerve-fibres outside. A hot and very thirsty person requir- 

 ing a big drink should therefore not take too cold water 

 or if he does, swallow it only a mouthful at a time. 



The blood-vessels of the alimentary canal are very numer- 

 ous and capacious and form one of the largest vascular tracts 

 of the whole Body, and through the reflex mechanism above 

 described we see how they may control the heart's beat. 

 Probably if the heart is beating too frequently and keeping 

 up too high a pressure in them, the sympathetic nerve- 

 fibres in their coats are stimulated and then, reflexly, 

 through the cardio-inhibitory centre slow the heart's beat 

 and lower the general arterial pressure ; and so we get one 

 co-ordinating mechanism by which the heart and blood-ves- 

 sels are made to work in unison. 



Some other afferent nerves are also known to be in con- 

 nection with the cardio-inhibitory centre. For instance, 

 some persons are made to faint by a strong odor, the olfac- 

 tory nerves exciting the cardio-inhibitory centre and stop- 

 ping or greatly slowing the heart. Deaths from the admin- 

 istration of chloroform are also usually brought about in the 

 same way, the vapor stimulating the sensory nerves of the 

 air-passages which then excite powerfully the cardio-inhibi- 

 tory centre and stop the heart. 



The Accelerator Nerves of the Heart. These originate 

 in the spinal cord, from which they pass by communicating 

 branches to the lowest cervical and upper dorsal sym- 

 pathetic ganglia and thence to the heart. When stimu- 

 lated they cause the heart to beat quicker, but under what 

 conditions they are employed in the physiological working 

 of the Body is not known. 



