622 



Comparative Morphology of Chordates 



ALLANTOIC 



VASCULAR 



PLEXUS 



Fig. 474. The circulatory system of a young swine embryo. The arteries are 

 shown in black; veins are stippled. All blood-vessels at this stage are paired, but 

 those of the left side only are shown in the figure. (Modified from Patten: "Em- 

 bryology of tbe Pig," Philadelphia, The Blakiston Company.) 



valves, eacli being a projecting crescentic thin fold of the wall of the 

 vessel that forms a small pocket so placed that backward pressure of 

 blood distends it (Fig. 475). Ordinarily there are three such valves 

 arranged in a set encircling the lumen of each arterial trunk. When 

 distended, the three meet and completely occlude the lumen. Between 

 each auricle and its ventricle are the relatively large atrioventricular 

 valves. Each consists of two or three extensive folds of the lining of 

 the heart (endocardium), so placed that under ventricular pressure 

 they swell outward into the passage and close it. To the free edges of 

 the folds are attached delicate tendons (chordae tendineae) which 

 are connected to short papillary muscles projecting from the wall of 

 the ventricle (Fig. 475). The chordae and their muscles prevent the 

 membranous folds from being forced entirely through the passage. 

 Usually the right valve consists of three flaps (tricuspid valve), while 

 in the left are only two (bicuspid or mitral valve). 



The venous sinus, a receiving chamber characteristic of the 

 hearts of fishes, amphibians, and reptiles (more or less reduced in 

 some reptiles), does not appear in the adult mammalian heart. But 

 in the embryo it is present, formed by confluence of the veins which 

 deliver blood into the auricle. Later it becomes unrecognizably merged 

 into the wall of the right auricle. In this matter of the venous sinus, 

 the hearts of birds and mammals are alike. In fact, so far as the adult 

 hearts are concerned, there appear to be no important differences 

 between mammalian and avian hearts. But in the relations of the 

 main arteries which serve as outlets from the ventricles and in the 

 embryonic history of the ventricles and their connections, there are 

 radical differences. 



