110 NORMAL HISTOLOGY AND .ORGANOGRAPHY. 



and pleura already described. These cells are plas- 

 tic, loosely attached to the subendothelium, and 

 form a slippery surface over which the arterial 

 blood flows rapidly. Any damage to these cells 

 results quickly in the formation of a small blood- 

 clot at the point of injury, from which we infer that 

 they play a most important physiological role in 

 their relation to the blood stream. The subendo- 

 thelium is made up of a delicate network of elastic 

 fibers, enclosing a few connective-tissue cells, which 

 allows the applied endothelium a limited amount of 

 mobility. The fenestrated membrane of Henle (called 

 the internal limited membrane of the media by 

 some authors) consists of a coarser elastic network 

 of heavier elastic fibers which when peeled away as 

 a whole presents on the exposed surface a basket- 

 work arrangement of its fibers with numerous in- 

 tervening elongated apertures like so many windows > 

 hence its name. This membrane in cross-section of 

 arteries appears as a wavy or corrugated white 

 line encircling the artery very near to its inner 

 surface. 



2. Tunica Media. This is the middle layer of 

 an artery, and makes up the bulk of its wall. In 

 small arteries a considerable amount of smooth cir- 

 cular muscle fibers is always present, while in the 

 larger arteries circular elastic and non-elastic con- 

 nective-tissue fibers make up its bulk. A sprinkling 

 of connective-tissue cells may be seen, also a limited 

 amount of longitudinal muscle and connective-tis- 

 sue elements. A few blood capillaries and lymphatic 

 spaces are present, which always connect with a 



