176 HISTOLOGY 



(Fig. 170, D) and has been exposed by cutting away most of the left 

 atrium and left ventricle. The septum between the atria becomes per- 

 forated as it develops, so that in embryonic life the atria always commu- 

 nicate. The perforation in the septum is the foramen ovale. 



Encircling the orifice which connects each atrium with the correspond- 

 ing ventricle, the is a ring of mesenchyma which in the adult becomes 

 dense fibrous tissue the annulus fibrosus. Extending from this ring 

 into the left ventricle there are two flaps of tissue partly detached from 

 the ventricular walls. They constitute the bicuspid valve (or mitral valve). 

 Toward the apex of the heart each flap passes into strands of tissue at- 

 tached to the walls of the ventricle. These strands become the chorda ten- 



D E F 



FIG. 170. EMBRYONIC HEARTS. "* 



A and B, From rabbits nine days after coitus; C, from a human embryo of three (?) weeks; D and E, from a 

 12-mm. pig (D sectioned on the left of the median septum, and E on the right of it); F, from a 13.6- 

 mm. human embryo, sectioned like E. The hearts are all in corresponding positions with the left 

 side toward the observer, the anterior end toward the top of the page, the dorsal side to the right, 

 ao., Aorta; c. s., coronary sinus; f. o., foramen ovale; i. f., interventricular foramen; 1. a., left atrium; 

 p. a., pulmonary artery; p. v., pulmonary vein; r. a., right atrium; s., septum membranaceum separating 

 the root of the aorta from tne right ventricle; s. c., coronary sulcus; v., ventricle; v. b., bicuspid 

 valve; v. t., tricuspid valve; v. v., vitelline vein; v. v. s., valves of the venous sinus. 



dinecB of the adult, and the muscular elevations into which they are inserted 

 are the papillary muscles (musculi papillares). The differentiation of 

 these structures has not taken place in the stage shown in Fig. 1 70. 



In the i2-mm. pig (Fig. 170, D) the median septum which has grown 

 up from the apex of the heart, so as to separate the right and left ventricles 

 from each other, is not complete. The ventricles still communicate 

 through the interventricular foramen, and through this aperture the blood 

 passes from the left side of the heart to enter the root of the aorta. The 

 root of the aorta is shown in E, a section of the same heart made on the 

 right of the median septum. The pulmonary artery and the part of the 

 aorta near the heart develop first as a single vessel; they become separated 

 from one another by the formation of a partition. As long as the dividing 



