150 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



arteriosus, a short, narrow portion of the heart-tube which is directed dorsally and 

 towards the left. The truncus arteriosus bends sharply upward and joins the 

 ventral aorta. 



The cavity of the heart approximately follows the center of the heart-tube. 

 Like the outside of the heart, it also is not of uniform diameter, but presents 

 swellings and constrictions, as shown in plate 3, figure 2. The sinus venosus passes 

 to the left as an irregular tube. It is marked off from the atrium by a slight con- 

 striction. At the apex of the atrial bend the cavity enlarges considerably. On 

 the superior surface of this enlargement is a crescentic projection, the concavity 

 of which is directed medially. This extends into the above-described projection 

 of the atrium as seen from the surface. 



Between the atrium and ventricle is a long, narrow portion of heart-cavity, 

 the atrio- ventricular canal. The difference in size between this narrow portion 

 and that of the adjacent swellings is greater than that between the same portions 

 of the heart as seen from its surface. The cavities of the ventricle and bulbus 

 cordis are again enlarged and separated from one another by a rather long con- 

 striction portion. This is again much narrower proportionally than the same 

 constriction seen from the surface of the heart. In cross-section the cavities of 

 the ventricle and bulb, particularly the former, are triangular. In the truncus 

 arteriosus the cavity is again small and irregular. Cephalad it gradually widens 

 out as it becomes the ventral aorta. 



The endothelium of the heart is everywhere a syncytium of one layer of cells. 

 Its nuclei are oval in shape and coarsely granular, while the cytoplasm stands 

 out sharply in contrast to the coagulum without. In the bulb and ventricle, at 

 the angles of the cavity as seen in cross-section, the endothelium sends out plate- 

 like processes of cells. These give to the various portions of the endothelial heart 

 a stellate appearance. In some instances these processes extend to the myocardial 

 layer and apparently fuse with it. One such process is shown in plate 4, figure 1. 

 It passes off from the endothelial tube of the heart as a solid cord of cells. It 

 extends entirely across the space between the endothelial and myocardial layers 

 and terminates in a bulbous expansion in contact with the myocardium. In the 

 expanded end is seen a distinct mitotic figure. The significance of processes of 

 this kind can not be doubted. They represent the earliest beginnings of the 

 so-called "heart sinusoids," which in embryos a little older are numerous. By 

 then- growth and anastomoses they give rise to the trabeculse of the heart. 



In the atrium, with the exception of the above-described atrial projection, 

 the endothelium lies in contact with the myocardium, there being no intervening 

 space such as is found in the remainder of the heart-tube. This relation of the 

 endothelium to the myocardium in the atrium has been noted by His 18 . The same 

 condition has been described by Mall 33 , who states: 



"This arrangement is so pronounced in the early heart that it affords a way by which 

 we may determine with precision the exact portion of the heart tube from which the 

 atrium arises." 



