128 



DE VEL OP MEN T. 



FIG. 97. Heart at the fifth week. 

 A. Opened from the abdominal aspect. 

 1. Arterial sinus. 2. Aortic arches 

 uniting behind to form the descending 

 aorta. 3. Auricle. 4. Auriculo-ven- 

 tricular orifice. 5. Commencing septum 

 ventriculorum. 6. Ventricle. 7. In- 

 ferior vena cava. B. Posterior view of 

 the same. 1. Trachea. 2. Lungs. 3. 

 Ventricles. 4,5. Auricles. 6. Diaphragm. 

 7. Descending aorta. 8, 9, 10. Pneumo- 

 gastric nerves and their branches. 



carried on between the foetus and the placenta by two arteries and one vein 

 (umbilicat). 



During the occurrence of these changes great alterations take place in the 



primitive heart and blood-vessels, above alluded 

 to, which will now require description. 



Further Development of the Heart. The sim- 

 ple median tube, formed by the coalescence of the 

 pair of tubes of which the primitive heart con- 

 sists, becomes elongated and bent on itself, so as 

 to form an S-shaped tube, the anterior part of the 

 tube bending over to the right, and the posterior 

 to the left. At the same time the middle portion 

 is protruded forward and arches transversally from 

 right to left and at the same time becomes twisted 

 on itself, so that the extremity from which the 

 arteries are prolonged is situated in front and to 

 the right, and that into which the veins enter is 

 behind and to the left. The bent tube then be- 

 comes divided by two transverse constrictions 

 into three parts. One, the posterior, becomes 

 the auricles, the middle one forms the two ventricular cavities, while the anterior 

 forms the aortic bulb, from which the commencement of the aorta and pulmonary 

 artery is developed. A division of each of these cavities now takes place, 

 so as to convert them into right and left ventricle, right and left auricle, 

 and aorta and pulmonary artery respectively. In the middle portion of the 

 tubular heart, the rudimentary ventricular cavities (Fig. 97, A, 5), a par- 

 tition rises up from the lower part of the right wall of this cavity, and 

 gradually grows up until it reaches the constrictions which separate it from the 

 other two, and thus the interventricular septum is completed. At the same time 

 a cleft appears on the outside, a little to the right of the most prominent point, 

 which ultimately becomes the apex of the heart. The cleft becomes less marked 

 as development progresses, but remains to some extent persistent throughout life 

 as the interventricular groove. 



The first appearance of a division in the posterior or auricular portion of the 

 tubular heart makes its appearance, at a very early period of development, in 

 the shape of two projecting pouches, one on either side ; these are the rudiments 

 of the auricular appendages, but the actual division of the cavity by a septum 

 does not occur until some time later. This is formed by the growth of a partition 

 from the anterior wall of the auricular cavity, which grows backward, and par- 

 tially separates the cavity into two. The partition, however, is not completed 

 until after birth, a part remaining undeveloped, and thus permitting of a com- 

 munication (foramen ovale) between the two auricles during the whole of foetal 

 life. In a like manner the aortic bulb is divided into two by the growth of a 

 septum downward, from the distal end of the bulb, which divides the cavity 

 into the permanent aorta and the pulmonary artery, and, uniting below with 

 the upper edge of the interventricular septum, places the aorta in com- 

 munication with the left, and the pulmonary artery with the right ventricle. 

 Very soon a superficial furrow appears on the external surface of this portion of 

 the heart corresponding to the septum internally, and, becoming deeper, the two 

 vessels are gradually separated from each other through the septum, in the imme- 

 diate neighborhood of the ventricular portion of the heart, whilst beyond this they 

 still remain joined together, and give origin to the fourth and fifth aortic arches, 

 presently to be described. 



Further Development of the Arteries. In the vitelline circulation two arteries 

 were described as coming off from the primitive heart, and running down in front 

 of the developing vertebrae. The first change consists in the fusion of these 

 arteries into one at some distance from the heart, thus forming the descending 



