261 



Nachdruck verboten. 

 The Question of Sinusoids. 



By Fkederic T. Lewis, A.M., M. D. 



(From the Embryological Laboratory, Harvard Medical School, 

 Boston, Mass., U.S.A.) 



With 10 Figm-es. 



In the Proceedings of the Boston Society of Natural History 

 (1900, Vol. 29, p. 185 — 215) Prof. Minot recorded some observations 

 "on a hitherto unrecognized form of blood circulation, without capil- 

 laries, in the organs of Vertebrata". He named the vessels of the 

 unrecognized form sinusoids. Prof, von Ebner, in Koelliker's 

 authoritative Handbuch (1902, Bd. 3, p. 664) concludes that the name 

 sinusoid is superfluous, as it designates merely a capillary of large 

 diameter. It is probable that von Ebner is not alone in thus inter- 

 preting Minot's sinusoids. I desire therefore to point out the essential 

 differences between capillaries and sinusoids, and to show that further 

 study has confirmed Prof. Minot's opinion that the recognition of si- 

 nusoids is of fundamental importance. 



A sinusoid may be defined as a subdivision of a vessel produced 

 by intercrescence between its endothelium and the parenchyma of an 

 adjacent organ. The proliferating tubules or trabeculae of an organ 

 encounter a large vessel and invade its lumen, pushing the endothelium 

 before them. The vessel, on the other hand, sends out branches to 

 circumvent the tubules. By the convolution or anastomosis of the 

 tubules or trabeculae, the large vessel becomes subdivided into small 

 ones. This is the process of intercrescence which produces sinusoids. 

 It follows that a sinusoidal circulation is either purely venous or purely 

 arterial. To demonstrate it for any organ, it must be possible to state 

 what vessel has been invaded and thus resolved into an afferent and 

 an efferent system. 



As a consequence of this mode of development there is an almost 

 entire absence of connective tissue between the endothelium of the si- 

 nusoids and the cells of the adjoining trabeculae. This is an essential 

 characteristic. 



Capillaries have a different history. The simple endothelial tubes 

 which form all the blood vessels of the embryo send out vascular 

 sprouts to ramify in the mesenchyma. The sprouts from a vein may 

 encounter those from an artery and anastomose with them. Such an 



