8 2 THE IN VOL UNTA R Y NER VO US S YSTEM 



muscle. In agreement with its histology the sinus venosus ex- 

 hibits a well-marked tonic rhythm which is influenced by the 

 cardiac nerves in the same way as that in the auricle. 



These observations definitely prove the existence in the heart 

 of a system of unstriped musculature in addition to the striated 

 cardiac muscles, which is especially well developed in the water 

 tortoises and extends from the auricles into the beginnings of the 

 great veins. This unstriped muscle resembles the enteral unstriped 

 muscles, the bronchial muscles, and those of the gall bladder in its 

 behaviour to poisons like atropine and muscarine and in its innerva- 

 tion ; for all these muscles are supplied with motor fibres from 

 cells connected with the vagus outflow, and with inhibitory fibres 

 from cells connected with the thoracico-lumbar outflow. On the 

 other hand the striated cardiac muscle resembles the enteral 

 sphincter muscles, in that their motor nerve cells are connected 

 with the thoracico-lumbar outflow, and their inhibitory cells with 

 the vagus outflow. In fact, exactly the same kind of reciprocal 

 innervation exists in the heart as in the intestine, and this fact 

 suggests in the mind of the observer the same question : were 

 the vagus and sympathetic cardiac nerve cells, which have 

 travelled out from the central nervous system, originally nerve 

 cells, whose axons divided into two nerve fibres, of which the one 

 was motor or inhibitory to the unstriped cardiac musculature, 

 and the other inhibitory or motor to the striated cardiac muscle ? 

 The disappearance of the unstriped muscle in the higher 

 vertebrates would bring about the disappearance of its motor 

 and inhibitory nerves, and leave the vagus cells inhibitory 

 to the heart muscle, and the sympathetic cells motor or aug- 

 mentor to it. 



It is an extraordinary thing that such a nerve as the vagus, 

 which is essentially the motor nerve to the striated musculature 

 of the respiratory portions of the gut, and supplies connector 

 fibres to the motor cells of the unstriped gut musculature, should 

 also be connected with motor cells to any cardiac musculature ; 

 but an explanation of the apparent anomaly is suggested when 

 we turn our attention to the manner of formation of the verte- 

 brate heart. 



Such a method of heart formation is unique in the animal 

 kingdom ; for the vertebrate heart is formed, as is well known, 

 by the coming together in the mid-ventral line of two vessels 



