THE FCETAL HEART. 



327 



pulmonary division of the bulb, and to send ramifications into the lungs, so as to form 

 the right and left branches of the pulmonary artery respectively : the farther or distal 

 portion of the right arch being obliterated, while the corresponding part of the left 

 side continued open, as the ductus arteriosus, until birth. According to this view, 

 the third arch on each side is persistent as the subclavian artery, and the external trunk 

 above this remains as the vertebral artery, and the internal as the carotid ; while the 

 internal trunk between the third and fourth arches of the right side becomes the 

 innominate artery. In so far as it applies to birds and some reptiles, this view may be 

 correct. But a different view of the metamorphosis, as it occurs in mammalia and 

 man, has more recently been presented by Rathke, which has been adopted by 

 Kolliker and others, and probably is more consistent with truth. According to 

 Rathke, in man and mammalia one arch only, viz. the left fifth, is concerned in the 

 formation of the pulmonary arteries; and the fifth arch of the right side is entirely 

 obliterated. From the fifth left arch a branch is given off, which, together with 

 the proximal part of the arch, forms the pulmonary artery, and which divides 

 into the primary branches for the right and left lung, the distal part of the arch being 

 converted, as according to Baer's theory, into the ductus arteriosus. The fourth arch 

 of the right side, according to Rathke, forms the commencement of the right subclavian 

 artery ; a branch is given off opposite the external extremity of the fourth arch on 

 both sides, which forms on the right side the remainder of the right subclavian, 

 and on the left the whole of that artery ; the vertebral arteries are derived from 

 the subclavians externally to the system of arterial arches ; the internal trunks in 

 their extent between the third and fourth arches remain as the common carotids, and 

 -in the remainder of their extent form the external carotids, while the third arches 

 and the external trunks above them are converted into the internal carotid arteries. 

 (Baer, Entwicklungsgeschichte, 1839 ; Rathke, Untersuchungen uber die Aortenwurzeln, 

 &c., Vienna, 1857, and Muller's Archiv. 1843, p. 276; A. Thomson, Edin. New 

 Philos. Journal, 1830-31, and Edin. Med. and Surg. Journ., No. 140 ; Ecker, Icones 

 Physiologicae ; Bischoff's works ; Kolliker, Entwicklungsgeschichte. 1861.) 



Fig. 246. 



PECULIARITIES OF THE F(ETAL HEART AND G REAT VESSELS. F<ETAL 

 CIRCULATION. 



Position. The foetal heart, even after all its parts are formed, continues to be 

 placed vertically in the thorax until about the fourth month, when the apex begins 



Fig 246. VIEW OP THE FRONT AND RIGHT 

 SIDE OP THE FOETAL HEART, AT FOUR 

 MONTHS, THE RIGHT AURICLE BEING LAID 

 OPEN (from Kilian). 



a, the right auriculo-ventricular opening ; 

 &, a probe passed up the vena cava inferior 

 and through the foramen ovale into the left 

 auricle ; c, vena cava inferior ; e, Eustachian 

 valve ; v, valve of the foramen ovale ; s, s', 

 vena cava superior. 



to turn towards the left side, so as to give 

 it an oblique position. 



Size. As compared with the body, the 

 heart is very much larger in the early 

 foetus than at later periods or subsequently 

 to birth. At one time, indeed, it occupies 

 nearly the whole thoracic cavity. At the 

 second month the proportion of its weight to 

 that of the body is said by Meckel to be 1 

 to 50 ; but the ratio becomes gradually re- 

 duced to that of 1 to 120 at birth. In the 

 adult the average is about 1 to 160. 



For a long period the auricular portion is larger than the ventricular, and the right 

 auricle is more capacious than the left; but towards birth these peculiarities disappear, 



