224 ESSENTIALS OF PHYSIOLOGY. 



The accelerator nerves also exert a tonic influence on the heart, tending 

 to quicken it, and when they are divided the heart beats more slowly. 



The rate of the heart is also modified by impulses passing to the vagus 

 centre from the higher parts of the brain, the acceleration occurring 

 during excitement, for example, being brought about by this means. 



The Action of Drugs on the Heart. The action of drugs on the 

 heart is most easily studied in the frog. The rate of the heart beat is 

 slowed by pilocarpine or muscarine, which stimulates the vagus endings 

 in the heart, this effect being abolished by atropine, which paralyses 

 these endings, so that after its administration stimulation of the vagi 

 has no effect on the rate of the heart. Atropine has no action on the 

 accelerator nerve endings. Nicotine first stimulates and then paralyses 

 the cell stations of the vagus in the heart ; and if it is painted on the 

 heart, stimulation of the vagus is ineffective, since the impulses passing 

 along it are blocked at the cell stations. Adrenalin stimulates the 

 nerve endings of the accelerator nerve, thereby increasing both the 

 force and the frequency of the heart beat. 



THE INNERVATION OF THE BLOOD-VESSELS. 



The Vaso-eonstrictor Nerves. If the ears of a rabbit, preferably 

 a light coloured one, are examined, it will be observed that when one 

 cervical sympathetic nerve is divided, the ear on that side almost 

 immediately becomes flushed. The central artery and its branches 

 can be seen to become wider, many small vessels previously invisible 

 come into view, and the whole ear becomes warmer than the opposite 

 one. Stimulation of the peripheral end of the cervical sympathetic 

 nerve causes an immediate constriction of the blood-vessels, many of 

 which disappear from view, and the ear becomes paler and cooler than 

 that of the opposite side. 



This experiment, which was first carried out by Claude Bernard, 

 shows that the cervical sympathetic nerve contains fibres which run to 

 the blood-vessels of the ear, and which, when stimulated, cause con- 

 striction of the arterioles by the contraction of their muscular walls. 

 It proves, further, that normally the muscular coats of the arterioles 

 are neither fully relaxed nor fully contracted, but are in a state of 

 partial contraction, which is spoken of as tone. The tone of the 

 arterioles exists only so long as they are in connection with the central 

 nervous system, and is dependent upon impulses passing from the 

 nervous system. The nerve fibres which carry these impulses to the 

 arterioles, and which when stimulated increase their tone, causing 

 them to constrict still further, are called vaso-constrictor nerves. 



In other organs the presence of vaso-constrictor nerves, and the 



