54 



PERICARDIUM, ENDOCARDIUM, VALVES. 



[The assumption that the muscles of the ventricle are arranged so as to form a figure of 8, or 

 in loops, seems to be incorrect ; thus, fibres are said to arise at the base of the ventricle, to pass- 

 over it, and to reach the vortex, where they pass into the interior of the muscular substance, to 

 end either in the papillary muscles or high up on the inner surface of the heart at its base. 

 Figs. C and D give a schematic representation of this view.] 



Only the general arrangement of the ventricular muscular fibres has been indicated. Accord- 

 ing to Pettigrew, there are seven layers in the ventricle, viz., three external, a fourth or 

 central layer, and three internal. These internal layers are continuous with the corresponding 

 external layers at the apex, thus one and seven, two and six. 



46 PERICARDIUM, ENDOCARDIUM, VALVES. The pericardium encloses within its two 



lavers* [visceral and parietal] a lymph space the pericardial space which contains a .small 



L r quantity of lymph the pericardial fluid. It 



has the structure of a serous membrane, i.e., 

 it consists of connective -tissue mixed with fine 

 elastic fibres arranged in the form of a thin 

 delicate membrane, and covered on its free 

 surfaces with a single layer of epithelium or 

 endothelium, composed of irregular, polygonal, 

 flat cells. A rich lymphatic network lies under 

 the pericardium (fig. 29) and endocardium ; 

 also in the deeper layers of the visceral peri- 

 cardium next the heart and between muscular 

 bundles (Salvioli). No stomata exist either 

 on its visceral or parietal layers. Around the 

 coronary arteries of the heart exist lymph- 

 vessels and deposits of fat, which lie in the 

 furrows and grooves in the subserosa of the 

 epicardium (visceral layer). 



The endocardium, next the cavity of the 

 heart, consists of a single layer of polygonal, 

 Fig. 29. flat, nucleated endothelial cells. [Under this 



Lymphatic of the pericardium, epithelium stained there is a nearly homogeneous hyaline layer 

 with nitrate of silver. (% 30, a), slightly thicker on the left side 



which gives the endocardium its polished 

 appearance.] Then follows, as the basis of the membrane, a layer of fine elastic fibres stronger 

 in the auricles, and in some places thereof assuming the characters of a fenestrated membrane. 



Between these fibres a small quantity of 

 connective-tissue exists, which is in larger 

 amount and more areolar in its characters 

 next the myocardium. Bundles of non- 

 striped muscular fibres (few in the auricles) 

 are scattered and arranged for the most 

 part longitudinally between the elastic 

 fibres. These seem evidently meant to 

 resist the distention which is apt to occur 

 when the heart contracts and great pres- 

 sure is put upon the endocardium. In all 

 cases where high pressure is put upon walls 

 composed of soft parts, we always find 

 muscular fibres present, and never elastic 

 fibros alone. No blood-vessels occur in the 

 endocardium (Langer). 



The valves also belong to the 

 endocardium both the semi-lunar 

 valves of the aorta and pulmonary 

 artery, which prevent the blood from passing back into the ventricles, and the 

 tricuspid (right auriculo- ventricular) and mitral (left auriculo- ventricular), which 

 protect the auricles from the same result. The lower vertebrata have valves in the 

 orifices of the venae cava?, which prevent regurgitation into them ; while in birds 

 and some mammals these valves exist in a rudimentary condition. The valves are 

 fixed by their base to resistant fibrous rings, consisting of elastic and fibrous tissue. 

 They are formed of two layers (1) the fibrous, which is a direct continuation of 

 the fibrous rings, and (2) a layer of elastic elements. , The elastic layer of the 



Fig. 30. 

 Section of the endocardium, a, hyaline layer ; b, 

 network of fine elastic fibres ; c, network of 

 stronger elastic fibres; d, myocardium with 

 blood-vessels, which do not pass into the endo- 

 cardium. 



