MORPHOGENESIS OF THE HEART AND PERICARDIUM 



525 



the coelomic cavity. The blended endothelial tubes form the endocardium. The splanclinlc 

 mesoderm in relation to the endocardium becomes the myoepicardium, and the double layer 

 connecting the heart dorsally and ventrally with the somatic mesoderm becomes the (temporary) 

 dorsal and ventral mesocardia. The somatic mesoderm of the heart region becomes the peri- 

 cardium. 



The originally straight heart-tube grows rapidly and becomes tortuous on account of its 

 increasing length between the hmits assigned by its fixed arterial and venous ends. Its arterial 

 end is continued into the truncus arteriosus, which is later divided into the pulmonary artery 

 and the ascending aorta. Its venous end receives the viteUine and umbiUcal veins, and, later 

 on, the common cardinals also. By the formation of a series of alternate bulgings and constric- 

 tions the heart becomes differentiated into the sinus venosus, atrium, ventricle and conus arte- 

 riosus, counting from the venous to the arterial end. These parts, after going through a process 

 of progressive differentiation and shifting (fig. 438) take up relative positions somewhat ap- 

 proaching those of the adult. 



Conus 

 arteriosus 



Ventricle 



Atrium 



Fig. 438. — Models showing the Development of the Heart. (After His.) 



Conus arteriosus Conus arteriosus 



t 



Ventricle' 



-- Left 

 atrium 

 Anterior 

 longitudi- 



— nal sulcus 



-Left 

 ventricle 



The sinus venosus lies on the dorsal wall of the atrium, and is composed of right and left 

 horns united by a transverse portion. The sinus is separated from the atrium by a sagitally 

 directed slit-like opening, guarded by right and left lateral valves which project into the atrium. 

 The atrium is wide, being prolonged into a ventrally projecting pouch on either side, the future 

 right and left auricles. The ventricle is situated caudal and somewhat ventral to the atrium. 

 The right limb of the common ventricle, which leads into the conus arteriosus, is the future 

 right ventricle; the left hmb, connected with the atrium, is the future left ventricle. The 



Fig. 439. — Sagittal Section through a Reconstruction of the Heart of a 9 mm. Human 

 Embryo seen from the Left Side. (Tandler, X 75.) 



Septum 

 ^^^""^•^""^ Foramen ovale 



Conus arteriosus' 



Atrial canal 



Septum prinum 



Sinus venosus 



Ventricle 



communication between the atrium and the ventricle, knowTi as the atrial canal, is indicated 

 on the exterior by a constriction; its interior consists of a transversely placed slit. The conus 

 arteriosus is continued from the ventricle without obvious constriction and passes over into the 

 truncus arteriosus. 



The sinus venosus early loses its bilateral symmetry owing to the rapid enlargement of the 

 right horn. This horn soon receives, through the proximal portion of the right viteUine vein 

 {inferior vena cava) , all the blood coming from the left vitelhne and both umbihcal veins. The 

 right common cardinal also gains ascendency over the left and becomes the superior vena cava. 



