158 UNCONSCIOUS NERVOUS OPERATIONS 



smooth, shining membrane, the endocardium* which also 

 covers the valves, and is continuous with the lining of the 

 veins and arteries. 



218. Cardiac Muscle. As stated in the chapter on Mus- 

 cles, the muscular iibers of the heart form a class by them- 

 selves, being striped but involuntary (Fig. 90). The 



fibers lie side by side, but send off at short 

 intervals branches which unite them. The 

 muscular fibers, moreover, are arranged in 

 the wall of the heart in bundles in such a 

 way that in contracting they draw the two 

 sides of the walls of the chambers together 

 until they meet. The muscle fibers of the 

 walls of the auricles are distinct from those 

 90 TWO ^ ^ e vehicles, so that th ev contract sepa- 

 cardiacmus- rately, as we shall see. Each fiber, or muscle 



cell, contains one nucleus. 



Cardiac muscle fiber appears, to a large extent, to origi- 

 nate its own contraction, and is not so entirely as is a skel- 

 etal muscle fiber a mere instrument of a motor nerve 

 fiber. The action of cardiac muscle under stimulus is not 

 stronger or weaker in proportion to the strength of the 

 stimulus, as is the case with skeletal muscles. A weak 

 electric shock, if it causes any beat at all in the heart 

 muscles, causes as strong a beat as does a strong stimulus. 



219. Arteries are the vessels which convey the blood 

 from the ventricles of the heart. The smallest of them 

 have a few plain muscular fibers wrapped round the tube 

 outside the endothelium. As the arteries grow larger, 

 the number of muscle fibers increases till they form a 

 definite muscular coat with a little connective tissue. In 

 the largest arteries the walls consist of three layers (Fig. 

 91) : (a) the inner coat, consisting of endothelium with a 



