CIRCULATORY ORGANS. 267 
the name implies, the capillaries are very small in diameter, but atten- 
tion has recently been called to the sinusoids, vessels with similar walls 
but larger in diameter, which are noticeable in some developing organs, 
especially the liver. Here also must be mentioned the retia mirabilia, 
places where an artery or vein suddenly breaks up into a network of 
small vessels (often capillary) which unite again, as in the glomeruli of 
the kidney, to form a vessel as large as before. In the lymph nodes 
there are similar networks of the lymph vessels. 
Arteries and veins (fig. 274) are larger than the capillaries and they 
have their walls strengthened outside of the intima by layers of smooth 
a 
Fic. 274.—Diagram of artery or vein. At the left the intima alone; covered in the middle 
by the muscularis, and at the right with the adventitia added. 
muscle fibres (muscle wall) and connective tissue, mostly elastic (ad- 
ventitial wall). Since the arteries are subjected to greater pressure 
than the veins their walls are relatively much thicker, but in other re- 
spects the two are much alike, except that valves to prevent the back- 
flow of the blood, may occur in the veins, especially those that are 
vertical in the normal position of the animal (legs). 
It has been suggested, with much plausibility, that the main blood-vessels are 
the remnants of the segmentation cavity, which elsewhere has been obliterated by 
the increase of the mesoderm. As will be recalled (p. 121) the mesothelium grows 
toward the middle line above and below the digestive tract, thus tending to narrow 
the segmentation cavity in these regions into two longitudinal tubes. ‘The epimeral 
part of the mesothelium divides into somites, and of course the segmentation cavity 
extends between these, and as these somites grow downward, these lateral exten- 
‘sions of the segmentation cavity are carried ventrally, so that at last they form a 
series of pairs of transverse vessels connecting the longitudinal trunks, thus forming 
the vessels of the somatic wall. Other tubes, connecting the dorsal and ventral 
trunks, would form between the two walls of the mesentery and between the 
splanchnic mesoderm and the entoderm, thus outlining the vessels of the alimentary 
tract. 
Even more speculative is the suggestion that the original circulation was lymph- 
oidal and that the blood circulation is a specialization of a part of this, the definitive 
lymph vessels being the unmodified part of the primitive system of vessels. 
