498 THE TISSUES. 



CHAPTER XII. 



THE VASCULAR SYSTEM. 



THE vascular system consists of the heart, the bloodvessels, and 

 the lymphatic vessels. 



I. THE HEAKT. 



The heart is a thick muscular organ, divided into four cavities, 

 covered externally by a serous membrane the pericardium and 

 lined internally by the endocardium, a continuation of the inner 

 tunic of the large vessels. 



The pericardium presents two layers; the outer (sero- fibrous layer) 

 being much the thicker, and fibrous in its external portion. The 

 inner layer, much thinner, is very intimately attached to the muscu- 

 lar fibres of the heart, except over the sulci containing the vessels 

 and nerves, where it is separated by common adipose tissue. Some- 

 times, however, the fatty sub-serous layer extends almost over the 

 whole heart. The scaly epithelium of the pericardium contains 

 one or two layers of cells, and presents no peculiarities. But few 

 lymphatics exist on the outer layer, while they are more abundant 

 in the muscular substance of the heart. Subdivisions of the dia- 

 phragmatic and recurrent laryngeal nerves have been demonstrated 

 by Luschka in the outer layer. 



The muscular fibres of the heart are of the transversely striated 

 kind, but they are about one-third smaller than those of the volun- 

 tary muscles (g^V o to isW f an inch), and are often more distinctly 

 striated in the longitudinal than the transverse direction. They 

 also almost always contain minute fatty granules, arranged in a 

 series in their axes. Their myolemma is very delicate, and often 

 not to be seen at all. The most striking peculiarity is, however, 

 the intimate union of the fibres ; they being everywhere separated 

 by a very scanty connective tissue, and never forming manifestly 

 distinct fasciculi. Besides, anastomoses of the fibres exist (Fig. 

 330), and also true divisions. 



For the complex course of the fibres of the heart, we refer to the 





