CH. ix.] INTRACARDIAC NERVES. 253 



passing down the vagi. As an example of this reflex stimu- 

 lation, the well-known effect on the heart of a violent blow on 

 the epigastrium may be referred to. The stoppage of the heart's 

 action in this case is due to the conveyance of the nervous impulse 

 by fibres of the sympathetic (afferent) to the medulla oblongata, 

 and its subsequent reflection through the vagi (efferent) to the 

 muscular substance of the heart. Chloroform vapour and tobacco 

 smoke in some people and animals, by acting on the terminations 

 of the vagi or their branches in the respiratory system, may also 

 produce reflex inhibition of the heart. Some very remarkable facts 

 concerning the readiness by which reflex inhibition of the fish's heart 

 may be produced were made out by Prof. McWilliam ; any slight 

 irritation of the tail, gills, mucous membrane of mouth and pharynx, 

 or of the parietal peritoneum, causes the heart to stop beating. 



In connection with the subject of reflex inhibition, it may be 

 mentioned in conclusion that though we have no voluntary control 

 over the heart's movements, yet cerebral excitement will produce 

 an effect on the rate of the heart, as in certain emotional conditions. 



Intracardiac Nerves. The heart beats after its -removal 

 from the body ; in the case of the frog and other cold-blooded 

 animals, this will go on for hours, and under favourable circum- 

 stances for days. In the case of the mammal, it is more a ques- 

 tion of minutes unless 'the heart is artificially fed through the 

 coronary artery with arterial blood. If this is done, especially 

 in an atmosphere of oxygen, the dog's heart, or even strips of the 

 dog's heart, can be kept beating for hours. (Porter.) At one 

 time this was supposed to be due to the intrinsic nervous system 

 of the heart ; the heart was regarded almost as a complete 

 organism, possessing not only parts capable of movement, but 

 also a nervous system to initiate and regulate those movements. 



We now, however, look upon the muscular tissue of the heart, 

 rather than its nerves, as the tissue which possesses the power 

 of rhythmical movement, because- muscular tissue which has no 

 nerves at all Assesses this property. For instance, the ventricle 

 apex of the frog's or tortoise's heart possesses no nerve-cells, but if 

 it is cut off and fed with a suitable nutritive fluid at considerable 

 pressure, it will beat rhythmically. (Gaskell.) The apparatus by 

 which this may be accomplished we shall study at the end of this 

 chapter. The middle third of the ureter is another instance of 

 muscular tissue free from nerves, but which nevertheless executes 

 peristaltic movements. Perhaps, however, the most striking 

 instance is that of the total heart, which begins to beat directly 

 it is formed, long before any nerves have grown into it. 



The power of rhythmical peristalsis therefore resides in the 



