^66 Dr Allen Thomson on the Vascular Si/stem 



weeks old * ; in that of the sheep of three weeks ; of the pig 

 at three weeks (Fig. 9- part I. and Fig. 37.), and of the rabbit 

 on the twelfth day ; and in the human embryo of six weeks 

 (Fig. 36.) : in the embryo of the dog, some little time before 

 that mentioned above, only three apertures are found. The 

 buccal opening situated anteriorly to the branchial clefts, the 

 inferior maxilla, the hyoid bone, and the opercular fold of inte- 

 guments, which closes the anterior clefts, are developed in the 

 same manner as in the bird. While three pairs of clefts exist 

 in the sides of the pharynx, there are in the dog (Fig. 35. 7»), as 

 in the chick, only four pairs of vascular arches ; but before the 

 first of these becomes obliterated, a posterior or fifth pair is 

 produced, while, at the same time, the fourth branchial cleft is 

 formed ; so that in the mammiferous animal five pairs of vascu- 

 lar arches, and four pairs of clefts, exist for some time simulta* 

 neously in the sides of the neck f. 



A few days after the appearance of the fifth arch, the neck 

 begins to elongate, the apertures are closed gradually on the out- 

 side, and the lower jaw becomes more developed ; while the 

 vascular arches undergo those changes by which the permanent 

 arterial branches, arising from the heart, are formed. 



The first and third pair of vascular arches form the carotid 

 and subclavian arteries in Mammalia (Fig. 39. t, w), as in birds, 

 and the second pair seems to be wholly obliterated, or at least 

 gives only a small branch ; in mammalia, however, the arch of 

 the aorta, or permanent communicating vessel between the as- 

 cending and descending aorta, is formed from the fourth branchial 

 arch on the left side (r) of the oesophagus ; so that the order in 

 which the vessels of the head and superior extremities arise is 



• See Fig, 35. the head of the fojtal dog represented by Baer, and given in 

 the first part of the Essay, which I have again inserted, in order that this, 

 interesting point of structure may be brought more immediately before the 

 eyes of the reader. 



•f The vascular arches of mammalia are described by Rathke and Baer in 

 the greater number of embryoes in which they have been seen, as simple 

 tubes ; but, in one instance, the latter author observed, on the internal and 

 concave border of each vascular arch another small vessel, of which, he says, 

 " je n*ai pas pu saisir les rapports." Could this have been the lateral vessel 

 whick) in the frog, gives off the smaller branches to the leaflets of the gill ? 



