114 THE VASCULAR SYSTEM 



tion of the flow in the himen and each cell has a nucleus in the 

 center of a clear cytoplasm. In fixed preparations, the capillaries 

 contract and in cross-sections the nuclei appear to protrude into 

 the lumen, a condition not true of living capillaries. 



Some investigators conclude that capillaries are intrinsically 

 contractile, and others believe that the contractility is due to 

 certain flat irregular cells, similar to reticular cells, which lie outside 

 the capillaries but in close contact with the wall. 



It is well known that plasma and leukocytes from the ]:)lood stream 

 pass out into tissue spaces through the thin capillary cell and also 

 that wastes in fluid form pass from tissue spaces into the stream 

 within the capillary. Some investigators claim that the exchange 

 is facilitated by minute openings where adjacent cells meet, but 

 in general the endothelial wall appears quite unbroken. The 



Fig. 65. — Sectional view of a capillary in the gill of a dogfish and erythrocytes within it. 



chemistry and physics of passage of fluids and white cells from the 

 capillary and back into it are incompletely known. 



Usually the blood-vascular system of a vertebrate is regarded as 

 a closed system in that the erythrocytes do not normally leave it. 

 In every other respect it is an open system. There is a constant 

 passage of food compounds from the intestinal tract into capillaries 

 and lymphatics in the villi where they lie near the epithelium. 

 Oxygen diffuses through the wall of the air sacs of the lung and is 

 taken up by erythrocytes in capillaries adjacent to the air sac wall. 

 Wastes of metabolism also pass from tissue juice into capillaries. 



The arrangement of the capillary network is determined to a 

 considerable degree by the disposition of the cells or tissue organiza- 

 tion supplied by the capillaries. Capillaries supplying skeletal 

 muscles are long tubules between adjacent nuiscle fibers and are 

 connected laterally. In the kidney there is a rich caiiillary network 

 between adjacent uriniferous tubules. In each villus of the small 

 intestine there is a basketwork of (•ai)illaries in the stroma just 

 within the epithelial wall. Secretory vesicles of the thyroid gland 



