S76 Dr Allen Thomson oji the Vascular Si/stem 



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 descending on the left side of the oesophagus: 

 The fourth arch on the right, and fifth on the left side, appeal* 

 to give rise to the pulmonary arteries. In the mammalia, thfe? 

 ductus botalli is formed, not as in birds or lizards, by the p^f*- 

 manence of the posterior part of the pulmonary arches, but by 

 a communication which remains in the bulb of the aorta between* 

 the roots of the pulmonic and systemic trunks *. Thus it is 

 explained how the aorta of birds corresponds with the right root 

 of this vessel in lizards, and that of mammalia with ' the Mt ;' 

 the arteria in^ominata of the left side being first given off iri 

 birds, while, in mammalia^ that on the right springs first froiri 

 the aorta. *> virrs '^i-jrbaa; Urkis 'nim y^yiil tii Anki 



XI. From these observations, it appears that it is erroneous 

 to compare the single heart of fishes or balrachia with the right 

 side or pulmonary cavities of the heart of higher animals. 

 They are similar, it is true, in this respect, that they both pro- 

 pel the blood into a respiratory organ ; but the relation of the 

 gills differs widely from that of the lungs to the heart ; and it 

 would be more correct to compare the single heart of fishes with 

 the whole heart of the higher animals though divided, or with 

 this organ in the early stages of their foetal development. 



XII. We have bad an opportunity of observing, that as we as- 

 cend in the series of vertebrated animals, the processes by which 

 respiration is carried on in the foetus, become gradually more 

 and more complicated. 1. The^ova of fishes are deposited and 

 developed in the same medium in which the adult animal conti- 

 nues to live : a. in osseous fishes, the blood is exposed to the 

 influence of the respiratory medium on the sac of the yolk ; 

 6. and ia cartilaginous fishes both on the yolk sac, and in exter- 

 nal gills. 2. The ova of batrachian reptiles are deposited, and 

 become developed in water ; while the animal, in its adult state, 

 breathes air. The blood in the larva or foetus of these animals 

 is arterialized by means of a yolk little developed, and by ex- 

 ternal and internal gills : and in the animal arrived at maturity, 

 by means of lungs, and a large urinary bladder or allantois. 



• Ttere is an approach to this form in the situctiife ol* 'tli6 vessels rising 

 from the heart in some of the saurian reptiles. '' 



