MINOT : SINUSOIDAL CIRCULATION. 
203 
from the blood only by the endothelium, and between the endothe¬ 
lium of two adjacent sinusoids are found only the specific cells of 
the organ, no connective tissue and no capillaries. 
When we study the development of the heart, we find that the 
above brief description is as strictly and literally applicable to the 
formation of the trabeculae of the heart as to the formation of 
the trabeculae (i. e., tubules) of the pronephros, mesonephros, or 
liver. The development of the cardiac trabeculae may be best 
studied in the ventricle, since they are very much restricted in the 
auricles. The heart, when first differentiated, consists of an inner 
endothelial tube, and an outer mesodermic (mesothelial) tube. 
From the latter arises the muscular wall of the heart; the muscular 
wall grows, and in the ventricle form a number of more or less sep¬ 
arate thickenings, which project into the cavity of the heart and 
constitute the anlages of the future trabeculae. From the very 
start, almost, the thickenings of the myocardium come into contact 
with the endothelium, which, as soon as the thickenings begin to 
protrude, forms a close investment over them. The intercrescence 
of the myocardium and endocardium continues, and there results a 
subdivision of the peripheral territory of the blood channel into 
many subsidiary channels. All of these facts are so well known 
that they constitute part of our text-book stock of information. I 
hold, however, in addition, that we must interpret this condition as 
a sinusoidal circulation, and as showing to us the first or primitive 
circulation of the heart itself. Later, when the coronary arteries 
develop, and connective tissue is differentiated in the ventricular 
wall, a true capillary circulation is established, just as in the liver 
the true capillaries appear ontogenetically late in the so-called “ pro¬ 
longations of Glisson’s capsule.” The adult heart has (? in all 
vertebrates) two circulations, the primary sinusoidal circulation and 
the accessory capillary circulation. 
That these conceptions are justified is proven by a more atten¬ 
tive study of the embryonic heart. Fig. 9 represents a small por¬ 
tion of the ventricle of the heart of a human embryo of about two 
months, and 24 mm. long. The muscular trabeculae, 1.1 ., are com¬ 
pletely and closely invested by the endothelium of the sinusoids, 
Si., Si. The trabeculae consist of young heart-muscle cells, and so 
far as I have observed contain no other cells whatever. I can find 
no connective tissue and no suggestion of capillaries. The principle 
of the organization is then the same as in the pronephros, meso- 
