No. 10. — On a hitherto unrecognized form of blood circulation 
without capillaries in the organs of Vertebrata. 
By Charles Sedgwick Minot, LL. D. 
This article is intended to draw attention to the fact that in all 
vertebrates there occur two types of blood-vessels with endothelial 
or endothelioid walls only. Vessels of both types intervene between 
the ends of the arteries and the veins entering the heart. The ves¬ 
sels of the first type are true capillaries and have the well-known 
characteristics of size and structure which are familiarly recapitu¬ 
lated in all text-books of histology. The vessels of the second type 
I propose to name “ sinusoids f' on account of their resemblance to 
true sinus and also to separate them clearly from genuine capil¬ 
laries. A sinusoid differs in many important respects from a 
capillary, although its walls also consist merely of an endothelial 
(or endothelioid) layer without any strengthening additions of 
adventitia or media. A sinusoid is of relatively large size, and its 
epithelium is fitted closely against the cells of the organ in which 
the sinusoid is developed, and it has numerous wide and free com¬ 
munications with the neighboring sinusoids of the organs; the 
sinusoid wall follows the shape of the parenchyma of the part, while 
a capillary follows its own shape, and is chiefly or wholly imbedded 
in connective tissue. A typical sinusoid has either no or exceed¬ 
ingly little connective tissue between it and the adjacent paren¬ 
chyma, and in those cases, where the connective tissue occurs, it is 
a secondary and late acquisition, and the amount remains usually, 
perhaps always, very small. 
The development of sinusoids in the embryo, so far as known, is 
essentially different from that of true capillaries. 
Blood sinusoids have been found to afford the main channels of 
the circulation in the following organs: — 
1. Pronephros. 
2. Mesonephros (Wolffian body). 
3. Liver. 
4. Heart. 
5. Supra-renal capsules. 
6. Parathyroid glands. 
7. Carotid glands (probably). 
8. Coccygeal glands. 
