990 DEVELOPMENT OF THE VASCULAR SYSTEM. 



grows more slowly, further, upon the side of the greater external pressure; and it 

 bends in the presence of one-sided pressure. 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



Heart The simple pouchlike rudimentary heart acquires the shape of the 



letter S (Fie 384 i) and soon exhibits a differentiation into the upper aortic 

 portion (a) with the bulb (6), the middle ventricular portion and the lower 

 venous portion (v} . The ventricular portion now bends upon itself in the shape of 

 the stomach (2) the venous portion in consequence taking a higher position (A) 

 and later on one somewhat back of the arterial portion. From the venous por- 

 tion' there grows to right and to left a blind sac, the forerunner of the large 

 auricle (7 o o ) The bend of the heart body corresponding to the greater curva- 

 ture (2, V) is divided externally by. a shallow groove into two large parts (3). 



a 



a 



FIG. 384. Development of the Heart (in Part Diagrammatic): i, First indication of the heart; a, aortic portion 

 with the bulb (&); v, venous portion. 2, Stomach-shaped flexure of the heart; a, aortic portion, with the 

 bulb (6); V, ventricle; A, auricular portion. 3, Development of the auricular appendages o, o\ and the ex- 

 ternal groove on the ventricle. 4, Beginning division of the aorta (p) into two longitudinal tubes (a). 5, View 

 from behind through the freely opened auricle (v, v) into the left (Z,) and the right (K) ventricle, between which 

 the septum projects, and in which respectively the two large arterial vessels (a) aorta and (*j) pulmonary 

 artery empty. 6, Relations of the entrance of the superior (Cs} and inferior (Cf) cavae in the auricles (diagram- 

 matic view from above); x, direction of the blood-stream from the superior cava into the right ventricle; 

 y, that of the blood-stream from the inferior cava into the left ventricle; tL tubercle of Lower. 7, Heart 

 of the mature fetus; R, right, L, left ventricle; a, aorta with the innominate artery (cc); carotid (c) and left 

 subclavian (5) artery; B, duct of Botal; p, pulmonary artery with the still diminutive pulmonary branches 

 i and 2. 



The large truncus venosus (4, v), which becomes embedded in the middle of the 

 posterior wall of the auricular portion, is formed by the union of the superior 

 and inferior cavae. Later, this common trunk is drawn into the wall of the 

 enlarging auricle, and in this way there result the separate openings for the 

 two cavae. In man the development of a special cavity in which the heart lies 

 occurs early; a portion of the rudimentary diaphragm bounds this cavity. 



Between the fourth and the fifth week the division of the heart into a right 

 and a left begins. In correspondence with the perpendicular groove in the ven- 

 tricle there first grows vertically upward into the ventricle a septum (5) that 

 divides the ventricular portion into a right and a left half ($,R,L). Between the 

 ventricular portion and the auricular portion the heart undergoes constriction, 

 forming the auricular canal. This contains a communication between the auricle and 



