210 PROCEEDINGS: -BOSTON SOCIETY NATURAL HISTORY. 
Comparison of capillaries and sinusoids. Capillaries and 
sinusoids differ from one another in three important respects, in 
size and shape, in their relations to other tissues, and in their 
development, and it is possible that they also differ in the character 
of their endothelial walls. We may review briefly these four 
points. 
1. Size and shape. Capillaries are of small diameter, and are 
usually too small to allow two blood corpuscles to move through the 
vessel abreast. Larger capillaries are frequently encountered, to be 
sure, but their calibre is never considerable. Sinusoids, on the 
contrary, are typically many times the diameter of a capillary; 
their dimensions vary between wide extremes ; in the liver by a 
secondary change the sinusoids are reduced in size until they 
resemble capillaries. Capillaries tend towards a cylindrical or sub- 
cylindrical form; sinusoids have irregular shapes and numerous 
irregular connections with one another. 
2. Relations to other tissues. A capillary is typically surrounded 
by and imbedded in connective tissue of mesenchymal origin. This 
can be clearly seen both in the embryonic and in the adult organs 
of all classes of vertebrates. When a capillary comes close to an 
epithelium, as in the villus of the intestine of a mammal, it still 
remains chiefly surrounded by connective tissue. In the lung of 
mammals the close adjustment of the capillary wall to the respira¬ 
tory entoderm is a secondary acquisition, and the meshes of the 
capillary network retain the connective tissue. Even those cases 
where a capillary seems partly or wholly imbedded in an epithelium 
(cf. F. Maurer, ’97.1, H. Joseph, ’98.1, Fr. Leydig, ’98.1, for interest¬ 
ing descriptions and discussions of intra-epithelial vessels) imply a 
condition which, it may be presumed, would be regarded as a sec¬ 
ondary acquisition by every one. A sinusoid has its endothelium 
closely fitted against the parenchyma of the organ. So far as yet 
observed in embryonic stages, there is no tissue of any kind between 
the endothelium and the organic parenchyma. A partial invasion 
of the organ by mesencliyma between the endothelium and the 
parenchyma may take place, as in the supra-renal capsule, or to a 
greater extent in the liver, and of course if the organ enters upon 
a retrogressive or degenerative development, the invading connec¬ 
tive tissue may become predominant, and completely alter the 
relations of the blood vessels, as in the parovarium. The relation 
of a sinusoid to connective tissue is certainly secondary. Nor does 
