274 Dr Allen Thomson on (he Vascular System 



iiianimalia, the bulb of the aorta is also divided, so as to enable 

 . each of the ventricles to communicate with those vessels only 

 into which they propel the blood during the whole of life. 



VIII. We have seen, that, in the early stages of development, 

 there is a uniform disposition of the greater arterial trunks in 

 all the orders of vertebrated animals, though the distribution of 

 these vessels is by no means the same as that which exists per- 

 manently. Tlie arteries arising from the bulb of the aorta, and 

 connected with the respiratory organs of the neck, have been 

 chiefly referred to, as affording one of the most remarkable ex- 

 amples of this uniformity of disposition in the vessels of the foe- 

 tus, and of the variety of transformation which they undergo 

 during their conversion into the permanent structure. 



IX. We have seen that, in all vertebrated animals, the anterior 

 intestinal part of the tube is encompassed by four or five * 

 pairs of arterial vessels, formed by the subdivision of the as- 

 cending aorta, and that these vessels, after passing round the 

 oesophagus, unite again with one another above this tube, and 

 below the vertebral column, to form the dorsal aorta. 



X. It has been seen, that, in the lower aquatic animals, gills 

 become developed along the course of parts of these vessels, 

 while in the higher or air-breathing animals, after being so dis- 

 posed as to indicate slightly the appearance of gills, these vessels 

 are gradually converted into the systemic and pulmonic arteries 

 by the processes of enlargement, partial obliteration, separation, 

 &c. Though the general phenomena occurring during this 

 transformation of the arteries in the neck, are analogous in 

 all vertebrated animals, there are certain remarkable differ- 

 ences respecting the obliteration of some, *and the perma- 

 nence of others of these vessels, in various species of animals. 

 1. In cartilaginous fishes, all the branchial divisions of the 

 aorta remain permanent to form gills, undergoing very minute 

 subdivision in these organs, so as to be converted into bran- 

 chial arteries and veins. 2. In osseous fishes, five pairs of 

 branchial arches are also observed in the foetus, but only four of 



• It appears probable that there are five in all vertebrated animals, ex- 

 cepting the lamprey, myxine, and some others. Baer has endeavoured to 

 demonstrate this in his essay on this subject in the 7th vol. of the Ile'pert. 

 Geiier. 



