VEINS. 127 



It is usually separated from the media by a more or less distinct 

 elastic membrane, the elastica externa. The fibres of the adven- 

 titia are divided roughly into two layers. The elastic fibres of 

 the inner layer, next the media, run circularly, while those in 

 the outer layer are longitudinal. Between the connective-tissue 

 coats of the inner layer are longitudinal bands of smooth 

 muscle. Vasa vasorum are present in the media and adventitia. 



Arteries of large calibre (e. g., carotid, aorta, etc.) cannot 

 be so distinctly divided into layers (Fig. 90). The endothelial 

 tube is made up of short polygonal cells. The intima consists 

 of a subendothelial connective-tissue sheath, and the elastica 

 inter na. The subendothelial sheath is made up of white connec- 

 tive-tissue fibrils and elastic fibres. The elastica interna is not 

 a firm homogeneous membrane, but is split up into several 

 lamellse, and in some places is only a simple layer of elastic fibres. 



The media contains a great many membrane-like masses of 

 elastic tissue, and thick elastic fibres. Between these are bun- 

 dles of smooth muscle fibres. The adventitia is similar to that 

 of medium-sized arteries. The elastica externa is wanting in 

 the aorta. 



The arteries in the skull cavity have no elastic elements in 

 the media. This perhaps explains why they are more likely 

 to yield to pressure than other arteries. The elastica externa 

 is not present in them, but there are circular elastic fibres in 

 the inner coat of the adventitia. 



(c) Veins. 



The important features which distinguish veins from arteries 

 .are the weak development of the media in the former, the 

 small amount of elastic tissue, and the strong development of 

 the adventitia. There is also a marked lack of uniformity in 

 veins of the same size. The same three coats may be spoken of 

 as in arteries, namely, intima, media, and adventitia. The 

 intima is a connective-tissue layer containing only a few elastic 

 fibres. In the larger vessels there are often bands of muscle 

 running in various directions, and a layer of elastic tissue 

 which may take on the form of a membrane. The latter is 



