236 Zoological Collections. Vertebrata. 



We have, in former numbers, referred to the essay of Dr A. Thomson, 

 on the development of the vascular system in the foetus of vertebrated ani- 

 mals, in which much valuable information will be found serving to the 

 elucidation of this doctrine of analogies. From this paper we extract the 

 following generalizations on the development of the great vessels connected 

 with the organs of respiration : — 



" In all vertebrated animals, the anterior part of the intestinal tube is 

 encompassed by four or five pairs of arterial vessels, formed by the sub- 

 division of the ascending aorta, and 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. 



" 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 disposed 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 remark- 

 able differences respecting the obliteration of some, and the permanence 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 branchial arteries and veins. 2. In osseous fishes, five pairs 

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

 remain to form the gills ; the anterior being partly obliterated, gives rise to 

 the roots of the carotid or head artery. 3. In batrachia, there is a gradual 

 transition from the structure of fishes to that of the higher reptiles. The 

 gills in the batrachia are, during some period of their existence, developed 

 along the course, or from particular parts of the branchial arches, in which, 

 as in fishes, minutely subdivided branchial arteries and veins are formed j 

 but these last gradually disappear, and more or fewer of the primitive 

 branchial vessels remain. a. In the batrachia with permanent tails, the 

 aorta is formed, as in the foetus, by the union of the whole four branchial 

 arches on each side, the pulmonary artery arising from the posterior arch ; 

 b. while in the batrachia without tails, as in the frog, only one branchial 

 vessel remains on each side, so as to form the right and left roots of the 

 aorta ; and the pulmonary artery, which in the foetus was given off from the 

 posterior branchial arch, appears to spring from the aortic root itself, in 

 consequence of the obliteration of the posterior part of the arch communi- 

 cating with the descending aorta. 4. Two branchial arches also remain 

 entire in the saurian and chelonian reptiles ; but in these, as well as in all 

 the other animals in which the ventricular part of the heart is more or less 

 divided in the progress of development, the pulmonary arteries — formed, as 

 in batrachia, by the posterior branchial arch — are separated from the aorta 

 and its branches ; each of these sets of vessels communicating directly with 

 its proper ventricular cavity. 5. In birds, the second pair of arches, and 

 the fifth arch of the right side, are wholly obliterated without giving rise to 

 any branches. The first and third form the arteriae innominatse, or carotid 

 and subclavian arteries on both sides, the communicating branches between 

 these arches and the roots of the aorta being obliterated at an early period. 

 The fourth arch on the right side alone remains entirely pervious during the 

 whole of life, and forms the proper trunk of the aorta from which the 

 innominatse spring. The fourth arch on the left, and the fifth on the right 

 side, united in a common root, give rise to the pulmonary arteries. These 

 arches remain pervious till birth, forming the ductus Botalli, or arterial ducts, 

 leading from the right ventricle into the aorta. 6. In mammalia, nearly the 

 same changes take place in the transformation of the anterior arches ; but 

 the aorta is formed in them by the fourth arch on the left side, this vessel 



