THE SINO-VENTRICULAR BUNDLE. 155 



disappearance of isotropic and anisotropic bands and a reduction in the amount 

 of sarcoplasm. The nuclei appear shrunken. The appearance is not at all unlike 

 a strand of collagenic tissue and undoubtedly many such degenerated muscle-cells 

 have been mistaken for connective tissue. 



It is therefore our opinion that the sino- ventricular bundle is to cardiac muscle 

 what the deep layer is to the superficial in transitional epithelium. It represents 

 the center of growth of heart muscle. It grows from the sinus region, the region 

 which is the most primitive of the heart, and by a series of cell divisions reaches the 

 ventricles of the heart to replace the worn-out heart-muscle cells. That is its 

 essential function. When we study the development of the heart (cf. Mall, 1915) 

 we readily see that the course of the bundle is predetermined by developmental 

 changes taking place in the primitive heart tube. In other words, the bundle takes 

 the shortest course possible between the sinus region and the ventricles of the heart. 



It has been pointed out that nerve fibers always accompany the bundle, and 

 thus far it has not been determined whether this accompaniment is due to physi- 

 ological association or is merely incidental. In the early embryos ganglion cells 

 are distinguishable in the atrial septum in the immediate vicinity of the bundle, 

 and it is but natural that their processes should take the same course as do the 

 bundle-fibers, because during development nerves always take the shortest course 

 to their sites of innervation. It is, however, significant that Wilson (1909), Morrison 

 (1912), and others have shown nerve terminations on the bundle. Basing it upon 

 their observations, there are some who are therefore inclined to argue that the 

 bundle is a specialized neuromuscular mechanism; but as the nature of these ter- 

 minations is not known and as there are likewise similar nerve terminations to be 

 found in other parts of the heart, it seems unwarranted to assume that there exists 

 a physiological difference between them. It is our opinion that these nerve termin- 

 ations have their origin in postganglionic neurones and most likely form a synapse 

 with the vagus terminations directly or through the intervention of another neurone. 



Most experimentalists agree that after section of the conductive system 

 stimulation of the vagus will not inhibit the ventricular beat. This bears out the 

 anatomical observation, namely, that the bundle is sinus tissue and naturally in its 

 growth down to the ventricles brings with it the nerve that is associated with it 

 viz., the vagus. I do not mean to imply that in these experiments vagus fibers 

 were necessarily cut. If the conduction passes by way of the sino-ventricular 

 bundle the very origin of the latter will explain the reason of the lack of response 

 to vagus stimulation upon the section of the area in question. The accelerator 

 fibers follow the course of the coronary vessels and are not anatomically associated 

 with the bundle. 



The work of Burrows (1912) dispels any doubt that may have existed that 

 embryonic mammalian heart-muscle has the power of rhythmic contraction. Em- 

 bryonic tissues have, however, greater potentiality than adult structures and it is 

 quite conceivable that in the adult the muscle-tissue has become so specialized that 

 it retains its power of contraction but has lost its power of conduction. If this is 

 the case, conduction takes place by nervous pathways, but it should be remembered 



