264 THE MECHANICS OF THE HEART 



with the adventitia of the large veins. Elastic fibers and a few smooth 

 muscle cells are also scattered through the endocardium, and espe- 

 cially through the lining of the auricles. As far as the function of the 

 pericardium is concerned, it may be stated that it exerts a restraining 

 influence upon the musculature of the heart, insuring a certain com- 

 pactness of its substance, and serving to counteract the effects of un- 

 usual degrees of pressure within its chambers. Thus, any defect in 

 this enveloping membrane generally permits of a decided outward 

 bulging of the cardiac substance which in turn may lead to an in- 

 competency of the valves. 



It is very suggestive that the heart of mammals is composed of a 

 type of muscle tissue which occupies an intermediate position between 

 the primitive smooth muscle and the specialized striated muscle. In 

 fact, its high content in sarcoplasm would tend to ally it more closely 

 with the former tissue. It is also of interest to note that in the lower 

 forms the cardiac muscle is composed of actual cells possessing a spindle- 

 like shape and an elongated nucleus. In these animals, the heart 

 appears essentially as a simple tubular muscle, the different parts of 

 which are intimately connected with one another by bridges of muscle 

 tissue. 



In the mammals, on the other hand, the mass of the ventricular 

 musculature is completely separated from the auricles by a heavy 

 deposit of connective tissue situated in the domain of the auriculo- 

 ventricular groove. It is to be noted that the perimysium enveloping 

 the muscle fibers increases very markedly at this level of the heart, 

 while the muscle fibers decrease in number, their places being taken 

 eventually by strong fibrotendinous rings, the so-called annuli fibrosi. 

 These structures which occupy the auriculoventricular furrow, 

 serve as the framework to which the different strands of muscle-tissue 

 are fastened. But, while the auricles and ventricles of the mammalian 

 heart are not united by direct bridges of muscle, they are brought 

 into functional relation by a strand of musculonervous tissue which is 

 known as the auriculoventricular bundle or the bundle of His. 



In accordance with the low degree of pressure developed by the 

 auricles, the musculature of these chambers appears as a thin capsule 

 to which, however, a seemingly disproportionate strength is given by 

 the musculi pectinati. These projecting strands of muscle tissue are 

 especially numerous in the domain of the appendix auriculae, where 

 they encroach upon the main cavity in such a manner that saccular 

 recesses are formed which are known as the foramina Thebesii. In 

 this way, the capacity of the central expanse of the auricular cavity, 

 which lies directly above the auriculoventricular orifice, may be greatly 

 increased at any time without incurring the danger of over-distending 

 and rupturing its wall. A circular depression upon the interauricular 

 septum indicates the location of the foramen ovale of intra-uterine life. 

 In addition, the right auricular cavity presents the orifice of the coro- 

 nary sinus, guarded by the delicate valve of Thebesius. In the left 



