186 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
I consider it not improbable that the circulation in the lymph 
glands, spleen, and in the thyroid gland of certain animals, is like¬ 
wise sinusoidal, but our existing knowledge seems insufficient to 
decide the correctness of my supposition. As regards the thyroid 
gland, I will mention in passing a suggestive observation made 
upon sections of the organ from kittens; in. these prejjarations the 
follicles are separated from one another by spaces, which are in 
part wide; these spaces are bounded by an endothelium, which is 
closely fitted against the epithelium of the gland. There seem to 
be no capillaries. Whether these endothelial spaces are blood 
channels or lymph chambers, I am unable to say at present. In 
the former case they are true sinusoids, as above defined. 
As to whether or not the 11 cavernous ” circulation of erectile 
tissue is sinusoidal or not, I am unwilling at present to express any 
opinion. 
Finally, I am inclined to add the placenta, at least in man and 
the rabbit, to the list of organs with sinusoids. Such a view must 
be critically tested, however, before it can be accepted definitively. 
This paper is a direct sequel to that which I published in volume 
28 of these Proceedings — see Bibliography at the end of this 
article, Minot, ’98.1. In the paper cited I called attention to the 
fact that the intertubular circulation of the pronephros and meso¬ 
nephros is not capillary, but—to use the term I now propose,— 
sinusoidal, — l. c., pp. 268-272. I will now briefly review the 
observations with which we are concerned, and which are, for the 
most part, more or less well known. The purpose is to show that 
the synthesis of these observations furnishes a secure generalization, 
namely, that there is an important type of circulation, distinct from 
the capillary. 
A word as to the illustrations! They are none of them strictly 
accurate as to the histological details. The outlines were drawn 
with the camera lucida, and are nearly correct, but the details were 
drawn in, free hand, without any attempt at real accuracy. In 
every case, however, the form of the blood spaces and their endo¬ 
thelium have been carefully reproduced. The magnifications given 
are nearly but not absolutely correct. 
1. Pronephros or Head kidney. In marsipobranchs the cir¬ 
culation of the pronephros, although it has not yet been very thor¬ 
oughly studied, appears to be of sinusoidal type. Wilhelm Muller, 
’75.1, p. 113, describes the tubules as lying partly in the venous sinus, 
