DEVELOPMENT OF THE HEART. 



323 



fractions of lines) are the average dimensions of these orifices in adult males and 

 females, between the ages of 20 and 60 years : 



Auriculo-ventricular orifices 



Right 



Arterial orifices . 



( Right (Pulmonary)... 

 ' (Left (Aortic) ......... 



Males. Females. 

 Inches and Lanes. 

 46 40 

 37 3 10 

 34 33 

 30 2 10 



DEVELOPMENT OF THE HEART AND GBEAT BLOOD-VESSELS. 



The Heart. The heart first appears as an elongated sac or dilated tube lying at 

 the fore part of the embryo, having two veins connected with it behind, and a 

 large arterial trunk proceeding from it in front. This tube exhibits rhythmic con- 

 tractions of its walls from a very early period. Its form is at first symmetrical, but 

 soon it becomes curved or bent upon itself like a horse-shoe, and projects on the 

 ventral aspect of the body towards the right side. 



Fig. 241. 



Fig. 241. VIEWS OF THE ANTERIOR OR 

 CEPHALIC HALF OF THE EMBRYO-CHICK 

 FROM THE ABDOMINAL SURFACE, SHOW- 

 ING THE HEART IN THE EARLIEST 



STAGES OF ITS FORMATION (after Re- 



mak). Magnified about twenty times. 



A, embryo after about twenty-eight 

 or thirty hours of incubation ; B, 

 after about thirty-six hours of incuba- 

 tion ; a, placed on the anterior cerebral 

 vesicle; b, the primitive cervical ver- 

 tebrae ; c, c, the cephalic fold of the ger- 

 minal membrane ; 1, 1, primitive veins 

 entering the auricle ; 2, 3, in A, the 

 primitive and simple sac or short tube of 

 the heart ; in B, 2, the auricular part ; 

 3, the ventricular part beginning to bulge 

 or be bent to the side and dilate ; 4, 

 the anterior part of the tube which be- 

 comes the aortic bulb. 



As this bending increases the venous end approaches the arterial, and at the same 

 time the tube, which progressively increases in size and in the thickness of its walls, 

 becomes divided by two slight constrictions into three compartments, opening suc- 

 cessively into each other. The first, next to the veins, is the auricular portion, the 

 middle one is the ventricular, and the last, which is the primitive arterial trunk, 

 is named the bulbus arteriosus. 



The auricular portion becomes placed behind the ventricular compartment, and 

 relatively to that cavity considerably enlarged. Moreover, two little pouches appear 

 upon it, one at each side, which form the future auricular appendages. The walls of 

 the ventricular portion are already thicker than the rest. 



The next series of changes consists in the gradual subdivision of the single auricle, 

 ventricle, and arterial bulb, each into two compartments, to form the right and left 

 auricles, the right and left ventricles, and the pulmonary artery and aorta; and these 

 changes are accompanied by an alteration in the position of the parts with relation to 

 the body, the ventricular portion now lying transversely, so as to bring that portion 

 which is afterwards to form the apex towards the left side. 



This subdivision commences first in the single ventricular portion of the heart. 

 A small notch appears externally to the right of the apex, which goes on increasing 

 in depth for some weeks, and then is again gradually obliterated. In the meantime, 

 about the fourth or fifth week, a septum begins to rise up internally from the right side 

 of the heart, at a little distance from the apex 'and from the anterior wall of the cavity, 

 and proceeds in the direction of the base, towards the arterial bulb, and about the 

 eighth week is complete. Traces of the subdivision of the auricular portion com- 



