134 HISTOLOGY. 



In the heart of a 12 mm. pig embryo the septum has formed (Fig. 

 159, D) and has been exposed by cutting away most of the left atrium 

 and left ventricle. The septum between the atria becomes perforated 

 as it develops, so that in embryonic life the atria always communicate. 

 The perforation in the septum is the foramen ovale. (The figure shows the 

 blind sprout of endothelium (p.v.} growing from the left atrium to form 

 the pulmonary veins.) Between the left atrium and ventricle the median 

 septum forms a flap-like fold; this and a similar fold from the outer wall 

 of the heart constitute the bicuspid valve [mitral]. The median septum 

 between the ventricles is never complete. It leaves an interventricular 

 foramen through which blood passes to the root of the aorta, which 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 across its lumen. As long as the dividing 

 wall is incomplete, the blood from either ventricle may pass out through 

 either artery as shown in E. In the more advanced human embryo, F, 

 the partition between the aorta and pulmonary arteries has extended 

 so that it joins the interventricular septum, and causes the interventricular 

 foramen to open into the root of the aorta only (s). 



The figures E and F further show that the veins which empty into 

 the right auricle unite to form the venous sinus just before terminating. 

 The outlet is guarded by a valve with right and left flaps. The left is 

 said to assist in the closure of the foramen ovale, which occurs at birth, 

 and leads to the formation of the fossa ovalis of the adult. The right flap 

 of the venous sinus forms the valve of the vena cava [Eustachian valve] 

 and the valve of the coronary sinus [Thebesian valve]. The coronary 

 sinus, Fig. 159, F, c.s. is the persistent terminal portion of a vein which 

 conveyed the blood from the left side of the embryo to the right atrium. 

 Most of its branches are lost by anastomosis with other vessels so that in 

 the human adult its territory is limited to the heart itself. It is found in 

 the coronary sulcus. Between the right auricle and ventricle is the tri- 

 cuspid valve, similar to the bicuspid in its development. These valves 

 are seen in section in Fig. 160. 



Embryologically the heart is composed of three layers, the endothelium, 

 mesenchyma, and mesothelium. The endothelium is continuous with that 

 which lines the blood vessels. The mesenchyma which surrounds it, 

 becomes in part differentiated into connective tissue which with the 

 endothelium makes the endocardium. In part it forms cardiac muscle, the 

 myocardium, together with the tendinous rings (annuli fibrosi) between the 

 atria and ventricles. As fibrous connective tissue it extends into the valves, 



