208 ANATOMY OF THE DOMESTIC FOWL 



called heart muscle. It is involuntary-striated and occupies an 

 intermediate position, both morphologically and embryologically, 

 between smooth involuntary muscle and striated voluntary muscle 

 (Fig. 74, No. 4). It, like striated voluntary muscle, is both trans- 

 versely and longitudinally striated. Heart muscle cells are short, 

 thick cylinders, which are joined end to end to form long fibers. By 

 means of lateral branches the cells of one fiber anastomoses with 

 cells of adjacent fibers. Each cell of heart muscle contains one 

 centrally located nucleus. There is no distinct sarcolemma, but 

 the sarcoplasm is more dense near the surface of the cell, which 

 gives it the appearance of an enveloping cell wall. There is a 

 zone free from fibrillae around the nucleus. The longitudinal 

 fibrillae, which make up the cell, are held together by a cement- 

 like substance. 



The main mass of the heart wall, called myocardium, consists of 

 the specialized muscular tissue just described. The myocardium 

 differs in thickness in different parts of the heart wall. It is thickest 

 in the left ventricle and thinnest in the auricles. The left ventricle 

 forces the blood through the systemic circulation and hence must 

 be thicker to give it more power than is needed for the right ventricle, 

 which forces the blood only through the lungs. The auricles are 

 thinnest of all; for they receive the blood and pass it only to the 

 chambers below. The auricular appendages at the base of the heart 

 in fowls are not so well marked as in mammals. The auricular 

 muscles consist of an outer coat common to both auricles, the fibers 

 of which are transverse and of an inner coat, independent for each 

 auricle, the fibers of which are longitudinal. Between the two 

 coats, occur bundles of muscle the fibers of which run in various 

 directions. The disposition of the muscle tissue of the ventricles 

 is much more complicated. It is composed of several layers of 

 fibers intricately interwoven. 



The endocardium, covering the inner surface of the myocardium, 

 forms a serous lining of all the chambers of the heart. At the 

 arterial and venous openings it is continuous with and similar in 

 structure to the intima of the vessels. The endocardium consists 

 of two layers, an external layer closely attached to the myocardium 

 and consisting of mixed fibers, including those of elastic tissue and 

 smooth muscle cells; and an inner, single layer of endothelial cells, 

 spoken of above. 



The heart is supplied with nutrient blood by the two coronary 



