SINUSOIDS. 



I2 5 



through the veins to the heart, and thence through the, arteries back to 

 the net. All of the future vessels of the body are believed to be offshoots 

 from the endothelial tubes just described. They grow out, as shown in 

 Fig. 149, through the mesenchyma with which they are inseparably 

 connected. The sprouts are at first solid but soon become hollow except 

 at the growing tips. They may encounter similar offshoots from the 

 same vessel or from other vessels and fuse with them. Through the 

 anastomosis of such sprouts, networks of vessels of small caliber are 

 produced which have been divided into two types, the sinusoid and capillary 

 types. 



Sinusoids are formed as branches or subdivisions of a single vessel. 

 A vein passing near a developing epithelial organ may send out branches 

 over its surface, and if the organ itself is a ramifying structure its sub- 

 divisions may be nearly enveloped by these venous branches. The liver 



VC.I. 



Int. V. Ar. 



FIG. 150. DIAGRAM SHOWING ON THE LEFT THE LIVER AND ITS SINUSOIDS; ON THE RIGHT THE PAN- 

 CREAS AND ITS CAPILLARIES. 



The connective tissue is represented by dots. Ar., Artery; Int., intestine; V., veins; 

 V. C. I., vena cava inferior; V. P., portal vein. 



is related in this way to the vitelline veins (in which the umbilical veins 

 later come to empty). In the left portion of the diagram, Fig. 150, the 

 liver is shown in heavy black as a branching outgrowth of the intestine. 

 The portal vein (V. P.), which is a persistent part of the vitelline veins, 

 forms a net of small branches, the endothelium of which is quite closely 

 applied to the hepatic tissue. A thin but important layer of connective 

 tissue intervenes, which could not be shown in the figure without great 

 exaggeration. The subdivisions of the portal (vitelline) vein are the 

 sinusoids and they come together to join the inferior vena cava, this* part 

 of which is also persistent vitelline vein. A relatively small hepatic artery 

 later supplies the connective tissue around the ducts of the liver, but the 

 essential vascular system of the liver is a single large vein which has been 

 resolved into a net of sinusoids. In the human adult, this is perhaps the 

 only instance of sinusoidal circulation. In the embryo the mesonephros 

 (a renal organ of large size) is supplied by sinusoids derived from the 



