THE HEART 45 



ventral wall of the primitive tube. The fusion of their adjacent 

 walls forms the interventricular septum. The top part of this 

 septum, therefore, represents the part of the primitive tube which 

 has been least disturbed by the evolution of the ventricles. Now 

 it is particularly interesting to note that it is at this point only in 

 the mammalian heart that the invaginated portion of the auricular 

 canal has persisted in the form of the A-V bundle. This point 

 alone proves the mode of development of the interventricular 

 septum. Further, not only has the bundle been subjected to the 

 least possible amount of disturbance by this process of develop- 

 ment, but in the adult heart it is undoubtedly better protected 

 here than in any other possible situation (Keith and Flack). From 

 this one infers that the A-V bundle is likely to vary but little 

 in its course and must have a very valuable function to perform, 

 since it is so well guarded both during development and in its 

 final form. 



The Bulbus Cordis. In Fig. 1 it will be seen that a fifth 

 chamber exists in the primitive vertebrate heart, the bulbus cordis. 

 This chamber is generally supposed to be absent in the mammalian 

 heart, but the recent researches of Greil and of Keith render it 

 probable that this is not the case. The infundibulum of the right 

 ventricle is the homologue of this portion of the heart. Of the 

 original musculature of the bulbus probably but little if any is 

 left, it having become replaced entirely or for the greater part by 

 that of the ventricle proper. There is therefore, as in the primitive 

 form, the freest muscular continuity in this part of the mam- 

 malian heart. This being so, we see that as in the primitive 

 cardiac tube we have in the mammalian heart free muscular 

 continuity from one end of the organ to the other from the 

 representative of the sinus venosus to that of the bulbus cordis. 



C. THE NERVOUS ELEMENTS OF THE VERTEBRATE HEART 



The vertebrate heart is undoubtedly very rich in nervous 

 elements. As considerable stress is laid by some observers upon 

 this fact, it is important to ascertain as far as possible their dis- 

 tribution. These elements may be classified as (a) ganglion cells ; 

 (6) nerve fibres and nerve endings. The ganglion cells are usually 

 regarded as the more important, since, as we shall see, the auto- 

 maticity of the heart is credited to them by some physiologists. 



