174 HISTOLOGY 



arteries. In certain veins (e.g., the main trunk of the portal, the renal 

 and suprarenal veins) the longitudinal muscle forms an almost complete 

 layer of considerable thickness (Fig. 168). 



The valves of veins are paired folds of the intima, each shaped like 

 half of a cup attached to the wall of the vein so that its convex surface 

 is toward the lumen. In longitudinal section they appear like the valves 

 of the lymphatic vessel shown in Fig. 179. The valves are generally 

 found distal to the point where a branch empties into the vein, and they 

 prevent its blood from flowing away from the heart. They are most 

 numerous in the veins of the extremities, but appear also in the inter- 

 costal, azygos, spermatic, and certain other veins; none are found in the 

 vertical trunks of the superior and inferior venae cavae. They counter- 

 act the effects of gravity upon the blood, and it has been suggested that 

 their arrangement in man corresponds rather to a quadrupedal attitude 

 than to an upright position. The endothelial cells on the surface of the 

 valve toward the lumen of the vein are elongated parallel with the current, 

 and beneath them there is a thick network of elastic tissue. On the side 

 of the valve toward the wall of the vein, the long axis of the cells is trans- 

 verse, and there the cells rest upon fibrous connective tissue. 



THE HEART. 



DEVELOPMENT. The heart has already been described as a median 

 longitudinal vessel situated beneath the pharynx, formed posteriorly 

 by the union of the vitelline veins, and terminating anteriorly in the 

 two ventral aortae (Figs. 27 and 28, p. 40). This endothelial tube is 

 surrounded by the mesothelium of the body cavity, except along its 

 dorsal border, where it is attached, as it were by a short mesentery, to 

 the under side of the fore-gut. If the embryo is placed in an upright 

 position, corresponding with that of the adult, the relations of the heart 

 to the body cavity will be as shown in the diagram, Fig. 169, A. The 

 posterior part of the body cavity, which becomes the peritoneal cavity, 

 extends forward on either side and comes together across the median 

 line beneath the heart, thus forming the pericardial cavity. As the heart 

 develops it becomes bent upon itself as shown in Fig. 169, B; and below 

 it, a shelf of tissue forms across the body, representing the future dia- 

 phragm. Dorsal to the diaphragm, the pericardial cavity still communi- 

 cates with the peritoneal cavity, on either side of the body. In the 

 region of this communication the lungs later develop, and partitions 

 separate the part of the body cavity around them, namely the pleural 

 cavity, from the pericardial and peritoneal cavities respectively. These 

 partitions are the pleuro-pericardial membrane and the membranous 

 part of the diaphragm (Fig. 169, C). Meanwhile the mesentery of 



