278 



has been established, the interesting question of the origin of the 

 vessels of Thebesius cannot be fully answered. 



In the frog's ventricle, the sinusoids open into a central cavity, 

 interrupting a thick connective tissue layer with which it is lined. 

 This is true also of the Torpedo, as suggested by Fig. IOC. In the 

 rabbit, however (Fig. 10 D), it will be found that a corresponding con- 

 nective tissue layer is hardly appreciable. The central cavity of the 

 ventricle is not distinct from the peripheral cavities of the sinusoids. 

 A comparative study of the development of the endocardium should 

 prove interesting. 



The sinusoids in the 21 days rabbit are therefore in part reduced 

 to slender vessels; in part they remain as broad shallow spaces be- 

 tween rounded columns of muscle. The myocardium has a compact 

 appearance, radically different from that of Amphibia, as recognized by 

 Hyrtl. Scattered through it are branches of the coronary arteries 

 and veins with their capillaries, reaching almost to the endocardium 

 which in the adult they penetrate. Thus it is brought about that the 

 coronary capillaries, which are primarily epicardial structures, become 

 buried in the myocardium. The sinusoidal circulation of the heart, 

 characteristic of embryos and lower vertebrates, in mammals becomes 

 vestigial. 



Conclusion. 



Sinusoids have been demonstrated histologically and embryologi- 

 cally in the Wolffian body, the myocardium, and the liver. To these 

 the pronephros should be added. Except in these organs sinusoids 

 have been determined only histologically. The histological or con- 

 nective tissue feature is of less importance, since as in the heart ca- 

 pillaries may have little connective tissue, and in the liver sinusoids 

 may become surrounded by it. 



The capillary development has been observed in the genital glands, 

 kidney, pancreas, lungs, and epicardium. In some important organs, 

 such as the suprarenal gland, the early development of the vessels is 

 unknown. 



It will be observed that sinusoids occur in primitive organs, the 

 pronephros and Wolffian body, but not in the kidney^); possibly in 

 the gills, but certainly not in the lungs. Moreover, although, they are 

 found in the adults of lower vertebrates they are regressive in mam- 

 mals. This justifies the opinion that the sinusoidal circulation is pri- 

 mitive. Its simplicity, since an organ is supplied by a single vessel 

 flowing near it, and not by the cooperation of two vessels with blood 



1) Cf. C. S. MiNOT, 1. c. p. 212. 



