BLOOD VESSELS 



167 



Where sinusoids are most highly developed, as in the liver and Wolffian body of 

 embryos, they possess another very significant characteristic. They are not con- 

 nections between an artery and a vein, like the capillaries, but are subdivisions of 

 veins. Thus in the liver, as shown in the diagram, Fig. 161, the portal vein enters the 

 organ and is subdivided by cords of hepatic cells into sinusoids, such as are shown in 

 section in Fig. 160. These reunite to empty into the vena cava inferior. The sinusoids 



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FIG 160. SINUSOIDS (Si) IN THE LIVER OF A CHICK EMBRYO OF ELEVEN ,DAYS. (Minot.) 

 h.c., Cords and tubules of hepatic cells. 



of the liver have therefore been described as formed by the intercrescence of vascular 

 endothelium and hepatic parenchyma. This arrangement of veins constitutes the 

 hepatic portal circulation, taking its name from the entering vessel. The same type of 

 venous circulation occurs in the Wolffian bodies, where it constitutes the "renal portal 

 circulation," although it has no connection with the portal vein. It is probable that 

 this form of circulation, which is generally lacunar or sinusoidal, represents a primitive 



VC.L 



V. Ar. 



FIG. 161. DIAGRAM SHOWING ON THE LEFT THE LIVER AND ITS SINUSOIDS; ON THE RIGHT THB PANCREAS 



AND ITS CAPILLARIES. 



The connective tissue is represented by dots. Ar., Artery; Int., intestine; V., vein; V. C. I., vena cava 



inferior; V. P., portal vein. 



type of vascularization, since a single vessel passing by or through an organ provides it 

 with both afferent and efferent vessels. The arterio-venous circulation requires the 

 presence of two vessels with currents flowing in opposite directions. There are indica- 

 tions that various organs in the human embryo have a transient "portal circulation" 

 before the arteries connect with the network and become the main afferent channels. 



CAPILLARIES. The capillaries are endothelial tubes of varying di- 

 ameter, the smallest being so narrow that the blood corpuscles must pass 



