THE HEART. 281 



OF THE VASCULAR SYSTEM. 



§ 212. 



The vascular system, consisting of the heart, with the blood- 

 and lymphatic vessels, contains in its interior the blood and 

 the lymph (chylus), with innumerable morphological particles. 

 The lymphatic vascular system presents special organs — the 

 lymphatic glands. 



1.— OF THE HEART. 



§213. 



The heart is a thick, hollow, muscular organ divided into 

 four compartments, invested externally by a serous membrane — 

 the pericardium, — and lined internally by the endocardium, a 

 continuation of the walls of the great vessels, particularly of 

 the tunica intima. 



The pericardium does not differ in structure from other 

 serous membranes, as, for instance, the peritoneum. The 

 outer lamella is considerably the thicker ; it is more fibrous 

 towards the exterior, presenting, towards the interior, numerous 

 fine, elastic networks, which are immediately covered with one 

 or two layers of tesselated epithelium. Very numerous elastic 

 networks of the same kind are found also in the inner thin 

 lamella which is, partly, very intimately united with the 

 muscular substance, and partly, especially in the sulci, sepa- 

 rated from it by common adipose tissue, which, moreover, not 

 unfrequently forms a subserous fatty layer, extending almost 

 over the entire heart. The vessels present the same conditions 

 as elsewhere ; and with respect to the nerves, twigs from the 

 phrenic and recurrent branch of the right vagus have been 

 demonstrated in the outer lamella of the pericardium (Luschka). 



The muscular fibres of the heart are red and transversely 

 striated, but differ in many respects from those of the volun- 

 tary muscles. The individual fibres themselves are, on the 

 average, about } more slender (0-004 — 01'"), frequently 

 more distinctly striated in the longitudinal than in the trans- 

 verse direction, and pretty readily divisible into fibrils and 



