i6o 



HISTOLOGY 



means of smooth muscle fibers which embrace the vessels. Longitudi- 

 nal fibers appear outside the circular layer. These muscle fibers are 

 the earliest additions to the vessel as it increases in size, and some 

 of the first of them, both circular and longitudinal, are indicated in 

 Figure 144. 



This muscle tissue is added to by elastic connective tissue and white 

 connective tissue in all larger vessels. In a well-developed artery these 



tissues are found 

 in three heavy 

 coats, each of 

 which is sub- 

 divided (Fig. 145). 

 The inner is known 



end. 



FIG. 144. Slightly oblique section of a very small artery in the 

 marrow of a Guinea pig. To the right, the endothelial lining 

 (end.) is most prominent, with the circular muscle nuclei (dr. mus.) 

 cut in transection. The cell outlines of the endothelial layer were 

 added from another preparation. A few inner longitudinal muscle 

 cells were present. X 700. 



as the intima, and 

 consists from 

 within outward of 

 the endothelial 

 lining, a thin, flat 

 layer of white con- 

 nective-tissue elements containing a few fine elastic fibrils, and a 

 smooth, even, and tough elastic membrane. The connective-tissue 

 layer is drawn out longitudinally into a thick reticulum, and the elastic 

 membrane, which is designated as the inner elastic membrane, is usually 

 thrown into longitudinal folds. 



Outside of this intima comes the thickest layer, the media. This is 

 made up of circular, smooth muscle fibers and elastic fibers in a varying 

 proportion, sometimes one and sometimes the other forming the greater 

 part of this layer. A few longitudinal fibers sometimes occur. 



The outermost layer is the adventitia, which begins as an outer elastic 

 membrane which much resembles the inner elastic membrane. When 

 longitudinal, smooth muscle fibers occur in an artery, they are placed 

 just outside of this line. Outside of this we find a thick mass of white 

 connective tissue that contains some fine elastic fibers. This layer acts 

 more as a means of attaching the vessel to the surrounding tissues and 

 carrying its blood and nerve supply than as a wall. 



A large number of variations may be seen in the other blood vessels. 

 Sometimes one layer is enormously developed or almost missing. Veins 

 as a rule are deficient in muscle, though some veins, on account of their 

 use, resemble arteries in their structure. 



Most vertebrates have two separate and distinct sets of blood chan- 

 nels, the one that we have been studying and a group of somewhat 

 smaller vessels known as the lymphatic system. This latter has a 

 separate and subsequent origin as a series of intercellular clefts which 



