DIVISION OF THE HEART'S CAVITIES. 7S7 



farther clown the dotted lines indicate tlie position of the succeeding arches to the number 

 of rive in all ; 5', 5', the continuation of the main vessels in the body of the embryo ; 

 6, 6, the omphalo-mesenteric arteries imssing out of the body of the embryo into the vas- 

 cular area of the germinal membrane. 



in width and most strongly bent upon itself, becomes the ventricular 

 portion ; and the third, situated anteriorly and retaining most the 

 simple tubular form, is the arterial or aortic bulb. This tubular stage 

 of the rudimental heart has been observed in the human embryo by 

 Coste and Allen Thomson (see fig. 585, A and B). 



Division into Single Auricle, Ventricle, and Arterial Bulb. — 

 By a continued increase of the inflection of the heart-tube, a change 

 in the relative position of the several parts is effected, so that the 

 auricular cavity comes to be placed above or behind (dorsally) and to 

 the left of the ventricular part, the veins being carried forwards along 

 with it, while the arterial bulb is attached by its extremity in front to 

 the neck of the embryo immediately behind the lower visceral plates. 

 There is as yet only a single passage through the heart, but the dis- 

 tinction of the auricular and ventricular cavities becomes more appa- 

 rent, both by an increase in the diameter of each, and by the constric- 

 tion which separates them, and by the much greater thickness acquired 

 ]:»y the walls of the ventricular and bulbous parts as compared with the 

 auricular portion. 



The three parts of the heart have now the appearance of being very 

 closely twisted together. The ventricular part becomes considerably 

 wider transversely, and the auricular part shows two projecting pouches, 

 one on each side of the arterial bulb, which are the first indications of 

 the future auricular appendages. At the same time the constriction 

 between the auricular and ventricular parts increases consideral:)ly, and 

 the constricted part elongating produces what has been called the 

 canalis auricular is. 



Division of the Cavities. Ventricles.— The next series of changes 

 in the developing heart consists in the division of each original single 

 cavity of the ventricle, auricle, and arterial bulb into two compart- 



Fig. 587. — Head op the Embrto of the Dog with the Fig. 587. 



Heart seen from below (from Kolliker, after BischofF). 



Magnified. <* 



a, cerebral hemispheres : h, eyes ; c, midbrain ; d, inferior 

 maxillary plates ; e, superior maxillary processes ; /, /', /", 

 second, third, and fourth branchial or visceral plates ; g, 

 right, h, left auricle of the heart ; h, right, i, left ventricle ; 

 1, aortic or arterial bulb, with three paii's of aortic or vascular 

 arches protruding from it. 



ments, so as to form the right and left ventricles 

 and auricles, and the stems of the -pulmonary 

 artery and aorta. The first of these changes 

 occurs in the ventricular portion, and is to be 

 seen in progress on the fourth day in the chick, 

 and the sixth and seventh week in the human 

 embryo. The ventricular chamber of the heart, 

 increasing considerably in breadth, that part of 

 it which ultimately becomes the apex of the 

 heart is thrown towards the left side, and in most mammals, and 



3 E 2 



