31 4 Dr Allen Thomson on the Vascular System 



corresponds to the auricle, the anterior to the ventricle and bulb 

 of the aorta. 



Though the circulating apparatus of the heart and vascular 

 area is apparently completed at the 35th hour, and a fluid re- 

 sembling blood is contained in the vessels, motion is not observed 

 till the 38th or 40th hour. AVhen the heart first begins to 

 move, it seems to contain only a colourless fluid and a few glo- 

 bules ; a slow and regular contraction then takes place in the 

 tube, commencing at the posterior, and extending gradually to 

 the anterior part. Very soon afterwards, the orange-coloured 

 flpid from the terminal sinus of the vascular area is carried into 

 the posterior part of the heart by the two venous branches al- 

 ready alluded to, and the circulation of the blood is then esta- 

 blished all over the vascular area. The vessels of the area now 

 become more defined, the colour of the blood deeper than be- 

 fore, and the heart contracts with increased velocity. 



From the 40th to the 50th hours, the foetus makes a slight turn 

 on its axis, so as to be placed with its left side towards the yolk. 

 In consequence of this change of position, the heart, which before 

 appeared to project from the right side of the foetus when viewed 

 from above, now hangs from the abdominal part of the body, to 

 which it is attached at its two ends. About the same time, the 

 simple tube, of which the heart has hitherto consisted, begins to 

 be slightly divided into several compartments ; the posterior part, 

 or auricle (45th hour, r), with which the veins {x x) communi- 

 cate, becomes dilated, and is separated from the anterior part as 

 well as from the veins, by a constriction in the paries ; the an- 

 terior part, or ventricle {s) becomes divided into two compart- 

 ments by a similar constriction in its canal ; the posterior com- 

 partment forms the ventricle itself, the anterior connects the ven- 

 tricle with the arteries, and constitutes the bulb of the aorta {t). 



Between the 50th and 60th hours, the circulation of the blood 

 on the vascular area becomes more vigorous, and the action of 

 the ventricle seems to succeed that of the auricle in a separate 

 period. The auricle increases in width laterally, the capacity of 

 the middle part or ventricle is also increased, and the bulb of the 

 aorta, lengthening itself, is applied more closely to the fore part 

 of the pharynx. The tube of the heart becomes more and more 

 bent together till it is doubled ; the auricle then passes below 



