VEINS 



187 



body. In epithelium and in cartilage there are no blood-vessels of any 

 kind, and in the splenic pulp it is doubtful if true capillaries occur. In 

 certain tissues large vascular spaces occur, which are comparable to 

 the capillaries in that their wall consists of scarcely more than the endo- 

 thelial tube, but which differ from the true capillaries in the extreme 

 size of their lumen. These vessels have been described by Minot (Jour. 

 Bost. Soc. of Med. Sc., 1900) as sinusoids. They are found in the erectile 

 tissues, adrenals-, coccygeal gland, parathyroids, in the maternal placenta, 

 and in the fetal liver, heart, pronephros, and mesonephros. They differ 

 from capillaries also in that they generally do not connect arteries and 

 veins, but are either exclusively arterial or venous. In the adult only 

 venous sinusoids occur. Retia mirabilia are capillary plexuses on ar- 

 terioles or venules; the best example of a rete mirabile in the human 

 body is the arterial capillary plexus on the efferent glomerular arteriole 

 of the kidney. 



VEINS 



The blood having passed the capillaries, enters the smallest radicals 

 of the venous system, the precapillary venules, and passes thence through 

 the venules to the larger veins. The pro- 

 gressive increase in the caliber of these suc- 

 cessive vessels is accompanied by a corre- 

 sponding increase in the thickness of their 

 wall. Thus, while the endothelial tube alone 

 composes the capillary wall, the endothelium 

 of the precapillary venule is encircled by a 

 delicate connective tissue membrane. In the 

 venule occasional smooth muscle fibers are 

 added to the wall of the smaller vessel, and 

 in the vessels of this caliber the fibrous tis- 

 sues have been so increased that the vascular 

 wall, as in the artery, can be said to possess 

 three coats. 



Precapillary Venules, The wall of the 

 precapillary venule consists of the endo- 

 thelial lining, which is surrounded by a 

 very delicate connective tissue membrane 

 in which are very few elastic and white 

 fibers. * 



FIG. 207. PRECAPILLARY 

 VENULE AND ARTERIOLE. 



The lighter nuclei are those 

 of the endothelium. The 

 darker nuclei in the venule 

 are in connective tissue cells; 

 in the arteriole they are in the 

 muscle cells. A, venule; B, 

 arteriole. Partly diagram- 

 matic. Highly magnified. 



