326 



THE HEART. 



disappear before the last are developed. This arrangement of blood-vessels, together 

 with the originally single condition of the heart, corresponds to a certain extent with 

 the permanent condition of the heart and branchial arteries in fishes ; with this 

 difference as regards the vascular arches in the human foetus and that of mammals, 

 birds, and scaly reptiles, that they never present any farther branchial subdivision. 



As the interventricular septum is approaching the base of the heart, that is, about 

 the seventh or eighth week, the arterial bulb becomes also divided by an internal 

 partition, meeting from opposite sides, into two vessels, which are slightly twisted on 

 each other, and are so adjusted as to become connected, the anterior with the right 

 and the posterior with the left ventricle : these vessels afterwards constitute the com- 

 mencement of the pulmonary artery and of the aorta. A furrow subsequently, begin- 

 ning on the outside, completes the separation into two vessels. 



Whilst the arterial bulb is thus converted into the commencement of the pulmonary 

 artery and. aorta, the five vascular arches arising from it undergo a metamorphosis, 

 by which the permanent aorta with the brachio-cephalic vessels and the pulmonary 

 arteries are formed. The general results of this change have been observed by 

 several ernbryologists, but it has not yet been made out with certainty in all its 

 details. 



It is generally admitted, however, that the fourth arch on the left side (counting 

 from above), which receives blood from the aortic division of the bulb, is persistent, 

 and, continuing to enlarge, eventually becomes the arch of the aorta. The fourth 

 arch on the right side, as well as the first, second, and third arches on both sides, 

 are obliterated to a greater or less extent, while certain portions of them, remaining 

 pervious and connected with the aortic arch, appear to form the commencement of the 

 great vessels rising from it. 



Both the arches of the fifth pair were held by Baer to be connected with the 



Fig. 245. Fig. 245. DIAGRAM OP THE AORTIC OR 



BRANCHIAL VASCULAR ARCHES OF THE 

 MAMMAL WITH THEIR TRANSFORMATIONS 

 GIVING RISE TO THE PERMANENT ARTERIAL 

 VESSELS (according to Eathke). 



A, P, primitive arterial stem or aortic 

 bulb, now divided into A the ascending 

 part of the aortic arch, and P the pul- 

 monary ; a, the right ; a', the left aortic 

 root ; A', the descending aorta. On the 

 right side 1, 2, 3, 4, 5, indicate the five 

 branchial primitive arterial arches ; on the 

 left side, I, II, III, IV, the four branchial 

 clefts, which, for the sake of clearness, have 

 been omitted on the right side. It will be 

 observed, that while the fourth and fifth 

 pairs of arches rise from the part of the 

 aortic bulb or stem, which is at first un- 

 divided, the first, second, and third pairs 

 are branches above c, of a secondary stem 

 on each side. The permanent systemic 

 vessels are represented in deep shade, the 

 pulmonary arteries lighter ; the parts of 

 the primitive arches, which have only a 

 temporary existence, are drawn in outline 

 only, c, placed between the permanent common carotid arteries ; ce t the external 

 carotid arteries ; ci, ci' the right and left internal carotid arteries ; s, the right sub- 

 clavian rising from the right aortic root beyond the fifth arch ; v, the right vertebral 

 from the same opposite the fourth arch ; v', s', the left vertebral and subclavian arteries 

 rising together from the left or permanent aortic root opposite the fourth arch ; P, the 

 pulmonary arteries rising together from the left fifth arch ; d, the outer or back part of 

 the left fifth arch, forming the ductus arteriosus j pn, pn', the right and left pneumo- 

 gastric nerves, descending in front of the aortic arches, with their recurrent branches 

 represented diagrammatically as passing behind, with a view to illustrate the relations of 

 these nerves respectively to the right subclavian artery and the arch of the aorta and 

 ductus arteriosus. 



p-n 



