802 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



The Heart, and Ductus Arteriosus. When the embryonic circulation 

 is first established, the heart is a simple tubular canal (Fig. 309), receiv- 

 ing the veins at its lower extremity, and giving off the arterial trunks at 

 its upper extremity. In the progress of growth, it soon becomes bent 

 upon itself; so that the entrance of the veins and the exit of the arte- 

 ries come to be placed more nearly upon the same horizontal level 

 (Fig. 310) ; but the entrance of the veins (i) is behind and a little 

 below, while the exit of the arteries (2) is in front and a little above. 

 The heart is then a simple twisted tube; and the blood passes through 

 it in a continuous stream, turning upon itself at the point of curvature, 

 and emerging by the arterial orifice. 



Fig. 309. Fig. 310. Fig. 311. 



f, 



Earliest form of the 

 FCETAL HEART. 1. 

 Venous extremity 2. 

 Arterial extremity. 



FOITAL HEART, bent 

 upon itself. 1. Venous ex- 

 tremity. 2. Arterial extre- 

 mity. 



FCETAL HEART, divided 

 into right and left' cavities. 

 1. Venous extremity 2. 

 Arterial extremity. 3, 3. 

 Pulmonary branches. 



Soon afterward, the single cardiac tube is divided into two parallel 

 canals, right and left, by a longitudinal partition, which grows from the 

 inner surface of its walls and follows the twisted course of the organ 

 itself. (Fig. 311.) This partition, which is indicated in the figure by a 

 dotted line, extends a short distance into the commencement of the 

 primitive arterial trunk, dividing it into two lateral halves, one of which 

 is in communication with the right side of the heart, the other with the 

 left. 



The pulmonary branches (s, z) are given off from each side of the 

 arterial trunk near its origin; and the longitudinal partition, above 

 spoken of, is so placed that both these branches fall upon one side of it, 

 and are both, consequently, given off from that 

 division of the artery which is connected with 

 the right side of the heart. 



The first portion of the arterial trunk is also 

 divided into two parallel vessels of nearly simi- 

 lar curvature, which join each other a short 

 distance beyond the origin of the pulmonary 

 branches. The left lateral division of the arte- 

 rial trunk is the commencement of the aorta (i ); 



HE ART still far- while its ri S ht lateral ^vision is the trunk of 

 ther developed. - 1. Aorta, the pulmonary artery (2), giving off the right 

 2 Pulmonary artery. 3, 3. anc ] j e f t pulmonary branches (s, a), at a short 



Pulmonary branches. 4. .... m . ,. ,. 



Ductus arteriosus. distance from its origin. That oortion of the 



Fi 



