CHAP. IX.] 



THE VASCULAK SYSTEM. 



Ill 



These sounds in certain diseases of the heart become changed 

 and obscure, and are replaced by various distinctive and charac- 

 teristic murmurs. 



The arteries. An artery is usually described as being com- 

 posed of three coats, an inner or elastic, a middle or muscular, 

 and an external or areolar. 



The inner coat of an artery consists of two layers : the inner 

 layer is composed of endothelium, 1 

 and forms a smooth lining for the 

 tube; the outer layer is a fine net- 

 work of elastic connective tissue 

 fibres. 



The middle or muscular coat con- 

 sists mainly of circularly disposed 

 plain muscular fibres. It has also 

 in most large arteries layers of elas- 

 tic fibres, which form close felted 

 networks, the fibres running for the 

 most part in an oblique and longi- 

 tudinal direction. 



The outer coat is formed of areo- 

 lar tissue, mixed with which are a 

 good many elastic fibres. The 

 strength of an artery depends largely 

 upon this coat; it is far less easily cut or torn than the other 

 coats, and it serves to resist undue expansion of the vessel. 

 The arteries are also protected by sheaths of connective tissue, 

 which surround and blend with the outer coat. 



By virtue of their structure, the arteries are both contractile 

 and elastic. The proportion of the muscular and elastic ele- 

 ments differs in different arteries; but, as a general rule, the 

 larger arteries are the more elastic, and the smaller the more 

 muscular. The elasticity and contractility of the arteries may 

 be demonstrated by the following example : 



If we tie a piece of a large artery at one end and inject fluid 



1 Endothelium is the name now generally given to the variety of epithelium 

 lining (i.e. lying within'} certain parts of the body ; it is composed of flattened, 

 transparent cells joined edge to edge so as to form smooth membranes. It is 

 found on the free surfaces of the serous membranes ; as the lining membrane of 

 the heart, blood-vessels, and lymphatics ; on the surface of the brain and spinal 

 cord, and in the anterior chamber of the eye. 



FIG. 85. STRUCTURE OF AN AR- 

 TERY. (Ledig.) A, internal coat, 

 with b, its inner layer of pavement 

 epithelium (endothelium); c, middle 

 coat, with transverse fibres; d, outer 

 coat, with longitudinal fibres. 



