168 THE FROG. 



The heart remains attached at its hinder end to the liver, 

 and in front to the floor of the pharynx ; but along the rest of 

 its length it becomes free, and increasing rapidly in length 

 becomes twisted on itself in a letter S shape. At the same 

 time, constrictions appear, partially dividing the cavity into 

 chambers, the first loop of the S forming the auricular, the 

 second the ventricular portion of the heart ; while the posterior 

 and anterior limbs become the sinus venosus and truncus 

 arteriosus respectively (Fig. 64). 



At the time of opening of the mouth the heart is still more 

 markedly twisted on itself, and the successive chambers more 

 sharply separated from one another; and a little later a septum 

 grows down from the dorsal wall of the auricle, dividing its 

 cavity into a small left auricle and a much larger right auricle. 



The condition of the heart in tadpoles of 12 mm. length is 

 shown from the right side in Fig. 76, p. 166 ; in sagittal section 

 in Fig. 65, p. 121 ; and in horizontal section in Fig. 71, p. 155. 

 The sinus venosus, or proximal division of the heart, is a wide 

 transverse vessel (Fig. 71, RS), which runs across the hinder and 

 dorsal part of the pericardial cavity, and receives the blood return- 

 ing from the body generally. 



The sinus venosus opens in front by a large round median 

 aperture into the right auricle (Figs. 65 and 71, RA). From the 

 auricle the blood passes through a narrow auriculo-ventricular 

 aperture (Fig. 65) into the ventricle, which receives also the 

 blood from the smaller left auricle. The cavity of the ventricle 

 is much subdivided by muscular trabeculas, which, growing 

 inwards from its walls (Fig. 65 and 75, RV), branch and unite to 

 form a kind of sponge work, in the meshes of which lie the 

 blood corpuscles. 



From the ventricle a small aperture leads into the truncus 

 arteriosus. This latter is divided into proximal and distal parts, 

 by a pair of valve-like folds, just before the point where it 

 bifurcates into right and left branches ; of these, the proximal 

 part, which becomes the pylangium of the adult, is partially 

 subdivided by a longitudinal fold, which runs along its interior 

 in a somewhat spiral course. It is difficult to imagine that 

 these valves can play any part in directing the blood into one 

 pair of afferent branchial vessels rather than another ; but it is 

 significant that they should appear just at the time when the 



