277 



remaining layers as free from capillaries as is the entire amphibian 

 heart. The sinusoids remain highly developed, as in the fishes. 



The heart of mammals develops like those already described, and 

 then undergoes changes which reduce its sinusoids to vestigial struc- 

 tures. This may be seen in the rabbit. Up to 9^/2 days the cardiac 

 vessel has a smooth lining. Then elevations arise in the ventricle and 

 a little later in the auricle. An embryo of 8 mm, 13 days, shows the 

 beginning of the coronary vein as a sprout from the venous sinus. It 

 subsequently branches forming the right and left veins, which have the 

 same disposition as in the Torpedo, and come to enter the venous sinus 

 separately (14 days, 11 mm). Although Martin states that the co- 

 ronary arteries appear on the 12th day, I failed to find them in em- 

 bryos younger than 14 days 18 hours. Then there were two indistinct 

 sprouts from the aorta just beyond the semilunar valves. At 16^2 

 days these arteries have extended to' the tips of the ventricles. In- 

 stead of running in the epicardium, as in lower vertebrates, they lie 

 embedded in myocardium. In single sections, Fig. lOD, the coronary 

 arteries, A. c, are indistinguishable from sinusoids since their con- 

 nective tissue investment is so slight. Except for the position of the 

 coronary arteries, the rabbit heart at this stage is much like that of 

 the Torpedo, Fig. IOC. The sinusoids have attained their maximum 

 development. 



In older rabbits there is a regression of the sinusoids in the ven- 

 tricle. At 21 days the muscle columns have come together, reducing 

 many of them to strands of endothelium without lumen. Others are 

 retained as slender vessels opening into the ventricle at both ends. 

 These are probably the source of those vessels of Thebesius described 

 by Langer ^), in adult human hearts, as associated with the papillary 

 muscles and communicating at both ends with the ventricle. Other 

 sinusoids remain large, and seem to anastomose with the coronary 

 vessels suggesting the free communication between the ventricles and 

 coronary veins found by Pratt 2) in the heart of calves. These con- 

 nections are very difficult to demonstrate in sections of embryos. All 

 of the vessels of Thebesius probably cannot be explained as persistent 

 sinusoids. Some which ramify in the endocardium appear like the 

 vasa vasorum of the large vessels. Until the embryology of the latter 



1) L. Langer, Die Foramina Thebesii im Herzen des Menschen. 

 Sitz.-Ber. d. Akad. d. Wiss. Wien, Bd. 82, 1881, p. 25. 



2) F. H. Pratt, The nutrition of the heart through the vessels of 

 Thebesius and the coronary veins. Amer. Journ. of Phys. , Vol. 1, 

 1898, p. 92. 



