J78 ESSENTIALS OF PHYSIOLOGY. 



bundle, or bundle of Si, the importance of which will be considered 

 later. 



The muscular fibres of the ventricles are arranged in a very complex 

 manner. A superficial stratum runs in a spiral direction from the 

 fibrous rings uniting the auricles and ventricles to the apex of the 

 heart; here the fibres form a whorl and then ascend in the inter- 

 ventricular septum and on the inner surfaces of the ventricles to end 

 in the papillary muscles. Between the layers thus formed are deeper 

 fibres, most of which are arranged in an CO -shaped manner, springing 

 from the papillary muscles of one ventricle and ending in the papillary 

 muscles of the other ventricle ; they are united by muscular strands 

 with the layers of the superficial stratum. The muscle is so arranged 

 that, when contraction occurs, the cavities of the ventricles become 

 smaller. 



The wall of the left ventricle, which drives the blood through the 

 greater part of the body, is about three times as thick as that of the 

 right ventricle, which drives the blood only through the lungs; the 

 thin-walled auricles merely discharge their contents into the relaxed 

 ventricles. The capacity of the two ventricles is approximately the 

 same, amounting in each case to a maximum of 140 c.c., and is rather 

 larger than that of the auricles 



The pericardium is a fibrous sac enclosing the heart, attached below 

 to the diaphragm and lined by flattened cells ; where the great vessels 

 pass through it the epithelial layer is reflected and covers the surface 

 of the heart. The smooth inner wall of the sac is moistened by a little 

 lymph (pericardial fluid), and the movements of the heart are carried 

 out with hardly any friction. The pericardium serves to prevent over- 

 distension of the heart, when it is being filled by the inflow of blood 

 from the veins. 



The Blood-vessels. The arteries, which convey blood from the 

 heart to the capillaries, are thick-walled tubes made up of muscular and 

 elastic tissue. A. medium-sized artery shows three coats outer, middle, 

 and inner. The outer coat is composed of fibrous tissue. The middle 

 coat consists of smooth muscle fibres arranged circularly, and of yellow 

 elastic fibres. The inner coat consists of flattened endothelial cells 

 united edge to edge by a cement substance, some loose connective 

 tissue, and a thick elastic lamina next the middle coat. The middle 

 coat of the large arteries, such as the aorta, contains a larger propor- 

 tion of elastic tissue and a correspondingly small amount of muscle, 

 whereas that of the small arteries (arterioles) is purely muscular. 



The capillaries form a dense network round and among the tissue 

 elements in almost every part of the body, and consist of a single layer 



