SECTS. 21 1, 211.] OF THE HEART. 477 



OF THE VASCULAR SYSTEM. 



§ 211. The vascular system consists of the Heart, the Blood- 

 vessels, and the Lymphatic Vessels; and in the cavities of these 

 structures arc contained the blood and the lymph (including the 

 chyle), with their innumerable morphological particles. The lym- 

 phatic glands, on the lymphatic system of vessels, are also to be 

 enumerated as special organs. 



I. — Of the Heart. 



§ 212. The heart is a thick muscular tube divided into four 

 compartments, surrounded externally by a serous membrane, the 

 pericardium, and lined internally by the endocardium, a continuation 

 of the walls of the large vessels, especially of their tunica intima. 



The pericardium resembles in its structure other serous mem- 

 branes, especially the peritoneum. The parietal layer is con- 

 siderably thicker than that which invests the heart; at its outer 

 surface it is more fibrous, but towards its inner aspect, as far as 

 the epithelium, it is provided with numerous networks of fine 

 elastic fibres. The epithelium is of the pavement variety, and 

 consists of one or two layers of elastic fibres. The visceral layer 

 of the pericardium is partly connected intimately with the 

 muscular fibres ; but in other parts, the grooves particularly, is 

 separated from them by ordinary adipose tissue ; which latter not 

 unfrequently appears as a subserous layer, covering almost the 

 whole heart. This layer is also abundantly supplied with elastic 

 fibrils. The vessels of the pericardium present the same arrange- 

 ments as in other parts ; and with regard to the nerves, branches 

 of the phrenic and right recurrent of the vagus have been demons- 

 trated in the parietal lamella (Lusehka). Papilliform processes, 

 like those on the pleura (see § 176), have been observed by Lusehka 

 upon the borders of the auricles. 



The muscular fibres of the heart are red and transversely 

 striated, yet differ in many respects from those of the voluntary 

 muscles. The individual fibres are, on an average, one-third thin- 

 ner (measuring 0*004'" to o'Oi"'), frequently more distinctly striated 

 in the longitudinal than in the transverse direction, and break up 

 with tolerable facility into fibrils and small particles (the sarcous 

 elements of Bowman). Their sarcolemma is very delicate, or even 

 may not be dcmonstiable at all except by the aid of re-agents. In 



