508 RIGHT VENTRICLE, / 



heart towards the base. Others of the columns arise from one 

 part of the surface of the ventricle, and are inserted into another 

 part. A third species are attached to the ventricle throughout 

 their whole length, forming ridges or eminences on it. The 

 columns of the two last described species are very numerous. 

 They present an elegant reticulated surface when the ventricle 

 is laid open, and appear also to occupy a considerable portion 

 of the cavity of the heart, which some of them run across in 

 every direction near the apex. They are all covered by a 

 membrane continued from the auricle and the tricuspid valves ; 

 but this membrane appears more delicate and transparent in 

 the ventricle than it is in the auricle. 



— ^This is called the internal serous, or endo-cardial lining 

 membrane of the heart. On the right side it is continuous with 

 that of the veins and pulmonary artery, on the left with the 

 aorta and pulmonary veins. It is extremely thin, smooth, and 

 transparent, covers all the interior surface of the cavities of the 

 heart, and by being thrown into folds, with some fibrous matter 

 interposed between the layers to increase their strength, con- 

 stitutes the valves. — 



A portion of the internal surface of the ventricle, which is to 

 the left, is much smoother and less fasciculated than the rest: 

 it leads to the orifice of the pulmonary artery, which arises 

 from it near the basis of the ventricle. This artery is very 

 conspicuous, externally, at the basis of the heart. 



It is very evident, upon the first inspection of the heart, that 

 the valvulaB tricuspides will permit the blood to flow from the 

 auricle to the ventricle ; but must rise and close the orifice, and 

 thereby prevent its passage back again, when the ventricle 

 contracts. 



The use of the tendinous threads, which connect the valves 

 to the fleshy columns, is also very evident ; the valve is sup- 

 ported by this connexion, and prevented from yielding to the 

 pressure and opening a passage into the auricle. The blood, 

 therefore, upon the contraction of the ventricle, is necessarily 

 forced into the pulmonary artery ; the passage to which is now 

 perfectly free. In this artery the membrane lining the ventricle 



