88 



ANATOMY FOR NURSES. [CHAP. VIII. 



The sounds of the heart. If the ear be applied over the heart, 

 certain sounds are heard, which recur with great regularity. 

 The first sound is a comparatively long, booming sound; the 

 second, a short, sharp, sudden one. Between the first and 

 second sounds, the interval of time is very short, too short to 

 be measurable ; but, between the second arid the succeeding first 

 sounds there is a distinct pause. The first sound is generally 

 supposed to be caused by the contraction of the ventricular 

 walls ; the second sound is undoubtedly caused by the sudden 

 closure of the semilunar valves. 



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 con- 

 sists of two layers : the inner layer 

 is composed of pavement epithe- 

 lium, and forms a smooth lining for 

 the tube ; the outer layer is a fine 

 network of elastic connective tissue 

 fibres. 



The middle or muscular coat con- 

 sists mainly of circularly disposed 

 plain muscular fibres. It has also 



FIG. 68. STRUCTURE OF AN , -, ^ < i 



ARTERY. (Ledig.) A, internal m most lar g e arteries layers of elas- 

 coat, with b, its inner layer of tic fibres, which form close felted 



pavement epithelium; c, middle 1^.1^1 _< ,1 



coat, with transverse fibres; a, networks, the fibres running for the 

 outer coat, with longitudinal mO st part in an oblique and longitu^ 



fibres. .*:. 



dmal direction. 



The outer coat is formed of areolar 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 con- 

 nective 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- 



