INHIBITORY NERVES TO THE VASCULAR SYSTEM 83 



which lie originally on each side of the notochord close against 

 the lining of the gut wall, which encloses the yolk. With this 

 infolding, by which the throat is formed and the two parts of the 

 heart brought together in the mid-ventral line, it is not impossible 

 that some of the muscular structures in reality belonging to the 

 gut may have become involved in the formation of the heart. 

 Such an assumption would account for the supply of motor 

 nerve cells of the vagus system to an unstriped musculature in 

 the heart, and also for the action of atropine and muscarine on 

 such a layer, while the varying character of the extent of this 

 muscular layer in the vertebrates and its disappearance in the 

 higher vertebrates would account for the final condition, in 

 which only motor or augmentor fibres from the cells of the 

 sympathetic system and inhibitory fibres from the vagal cells 

 remain. 



In close connexion with this question of the presence of 

 inhibitory nerve cells and nerve fibres to only certain muscular 

 tissues in the heart, is the striking difference between different 

 animals in the behaviour of their hearts to stimulation of the vagus 

 nerve ; for whereas the inhibitory action of the vagus can be de- 

 monstrated on the musculartissue of the sinus, auricles and ventricle 

 of all the Amphibia, such action is confined to the musculature 

 of the sinus and auricles in the Reptilia. This statement is founded 

 on the behaviour of the tortoise, snake and crocodile in none of 

 which have I ever seen any inhibitory effect on the ventricular 

 muscle produced by stimulation of the vagus nerve. In this 

 respect the mammalian heart behaves like the amphibian and the 

 avian like the reptilian. According to McWilliam the eel's 

 heart behaves like that of the tortoise. The meaning of this 

 strange difference in various vertebrate hearts which have been 

 investigated is a very difficult problem and will be discussed later. 



The nerve cells in the heart are everywhere associated with 

 the course of the vagus fibres ; thus in the frog the cells known 

 as Remak's ganglia are massed, where the fibres first enter the 

 heart along the superior vena cava ; again in connexion with 

 the two vagus nerves in their course along the auricular septum 

 we find the inter-auricular ganglia or Ludwig's ganglia, and 

 finally the vagus nerves terminate in the two ganglia at the 

 auriculo-ventricular junction, known as Bidder's ganglia. A 

 few ganglion cells have been found by Dogiel and others in the 



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