794 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



obliterated. Each of tlie first four branchial arches occupies a place in 

 the substance of the pharyngeal or visceral plates, and in front of one 

 of the pharyngeal clefts. The first or anterior is therefore situated in 

 the inferior maxillary plate, and in front of the tympano-Eustachian, or 

 first pharyngeal cleft ; and the fifth arterial arch is placed behind the 

 fourth pharyngeal cleft, and in the substance of the neck, in which there 

 is no distinct bar or plate in the higher animals, but which is the seat 

 of a developed branchial bar in some aquatic animals. 



The vessels forming the arterial arches are given off on each side in 

 succession from two short canals, into which the primitive arterial bulb 

 divides immediately in front of the place where it joins the neck. 

 These may be named the lower (ventral) or anterior aortic roots ; and 

 similarly, when they have passed round the wall of the pharynx, the 

 branchial arches unite in succession into a vessel on each side, thus 

 forming the upper (dorsal) or posterior aortic roots. 



On the third and fourth days in the chick, and from the fourth to 

 the sixth week in the human embryo, there are still three complete 

 pairs of arterial arches passing round the pharynx, and connected both 

 before and behind with the anterior and posterior aortic roots previously 

 mentioned. The transformations of these arches were in part traced 

 by Von Baer and various other observers, but the fuller knowledge of 

 their changes is due to the later researches of Eathke (Mem. of 



Fig. 593. Fig. 593. — Diagram of the Aortic or 



., Branchial Vascular Arches op the 



Mammal, with their transformations 



GIVING RISE TO THE PERMANENT ArTERIAL 



Vessels (accordiug to Ilathke, slightly 

 altered). 



A, P, ijrimitive arterial stem or aortic 

 Lnlb, 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 

 Ijranchial 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 com- 

 mon carotid arteries ; ce, the external carotid arteries ; ci, c'l, the right and left internal 

 carotid arteries ; s, the right subclavian rising from the right aortic root beyond the 

 fifth arch ; r, 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)n, 2^n , the right and left pneumogastric nerves, descending in front of the aortic 

 arches, with their recurrent branches represented diagrammatically as passing behind, 

 with a view to illustrate tlie relations of these nerves respectively to the right subclavian 

 arteiy (4) and the arch of the aorta and ductus arteriosus {d). 



