THE HEART AND VASCULAR SYSTEM. 173 



STRUCTURE OF THE HEART. The heart is composed of finely 

 striated muscular walls, with external serous covering from the 

 pericardium and internal serous lining the endocardium, the 

 latter continuous with that of blood-vessels. The endocardium 

 is a thin, translucent membrane, consisting of endothelium and 

 a fine basement membrane, beneath which is a fibre-elastic layer. 

 The doublings of these layers constitute the valves. The tricuspid 

 and mitral valves, as well as the aortic and pulmonary, are 

 surrounded by a fibro-elastic ring, which furnishes a "punctum 

 fixum" for the various muscles of the heart. 



The muscular structure consists of an intricate interlace- 

 ment of fibrous bands. Of these there are two groups those 

 of the auricles and those of the ventricles. 



The former consist of a superficial transverse set and the 

 internal or deep set, of which there are the looped and the 

 circular. 



The fibres of the ventricles consist of seven layers in a gen- 

 eral oblique and circular manner, and terminating in a whorl 

 or vortex at the apex, some of the fibres terminating in the 

 columna3 carnese, musculi pectinati, while others ascend, form- 

 ing in their course a twisted loop like 8. The arteries of the 

 heart are the right and left coronary. The veins are the an- 

 terior or great, middle or posterior cardiac, the left auricular, 

 the right auricular, and venae Thebesii. The lymphatics end in 

 thoracic and right lymphatic ducts. The nerves are derived 

 from the cardiac plexus of pneumogastric, spinal, and great 

 sympathetic. 



THE VASCULAR SYSTEM. 



The vascular system consists of four sets of vessels arteries, 

 capillaries, veins, and lymphatics. Anastomoses are common. 



The vessels that convey blood to and from the tissues of the 

 body generally constitute the general system. 



Those that carry blood to and from the lungs form the 

 pulmonary system. 



The vessels passing to the liver form the portal system. 



THE ARTERIES, for the most part, are composed of three 

 coats : 



Tunica adventitia, or external coat, consists of fibrous tissue, 

 thinnest on the largest trunks, and disappears in those which 

 merge into capillaries. 



The tunica media, or middle coat, is thickest in the large 

 trunks, and consists mainly of elastic tissue, together with some 

 unstriated muscle and some connective tissue; in smallest ar- 

 teries it consists alone of muscular tissue. The elastic tissue 



