THE ADRENAL SECRETION AND THE HEART. 437 



portions of the muscular substance of the heart, although it is 

 well known that, upon the surface and in the connective-tissue 

 interstices, arteries are found with their venae comites, just as 

 they are in other organs and tissues. The arrangement of the 

 vessels upon the surface and in the interstices is in marked 

 contrast with that found in the muscular substance proper. 

 Here, when an arteriole is accompanied by an efferent vessel, 

 this vessel is single coated and composed of endothelium, being 

 exactly like the smallest capillaries, except in size. These pecul- 

 iar large capillaries are found not only in company with arteri- 

 oles, and, therefore, when carrying on the function usually 

 performed by veins, but also alone. When they are alone, it 

 is impossible to be certain whether their function was afferent 

 or efferent. It may well be that arterioles are less numerous 

 in the heart than in other tissues, and that their place is taken 

 by the large capillaries. These capillaries are so numerous and 

 of such size that it seems likely they perform the function of 

 reservoirs. The presence of the large capillaries and the pene- 

 tration of the muscular fibers by the capillaries indicate a pro- 

 vision for the blood-supply of the heart even more bountiful 

 than that of the other organs." 



That these vessels are the Thebesian channels is evident; 

 their mode of distribution and the peculiar endings of their 

 subdivisions is particularly well shown in Fig. 6, while the 

 outpouring of plasma for absorption by the muscle-elements is 

 suggested by Fig. 5. 



The fact that the distribution of the Thebesian channels 

 is analogous to that observed in the heart of the frog has, we 

 have seen, been noted by Pratt. This had also been noticed 

 by Lannelongue, but this author considered the channels of the 

 human myocardium as vestiges of the batrachian system. Ber- 

 dal, who alludes to the latter, states that in the frog and in the 

 batrachian urodela there are no ordinary blood-vessels. "The 

 muscular fasciculi intercept cavernous spaces into which the 

 blood penetrates directly and from which they are only sepa- 

 rated by endothelium. The frog's heart is thus a true sponge the 

 structures of which, formed of muscular fibers, nourish themselves 

 ~by imbibition. In mammals, on the contrary, the myocardium 

 contains distinct vessels. The capillaries form a net-work the 



