^60 Dr Allen Thomson on the Vascidar System 



the fifth arch on the left side becomes less, gradually carries a 

 smaller quanj:ity of blood, and soon after the separation of the 

 aortic and pulmonary roots is entirely obliterated. Three arches 

 only, therefore, now remain, viz. the fourth on the right side or 

 the trunk of the aorta itself, its corresponding arch on the left 

 side, and the fifth on the right, the two last of which soon after j 

 #6.,% to,t|ie pulmonary arteri^?,^^ .,^, ^Wb h^li^tMm (« -A 

 ^ On the fifth and sixth days, the parietes of the pharyngeal or^ 

 branchial cavity of the chick also undergo a transformation. Ac- 

 cording to Rathke, the part intervening between the mouth and 

 the first pair of branchial apertures becomes thicker and firmer,, 

 and is divided by a transverse gro6ve into two portions (Figs.' 

 :24 and 25.) The anterior of these (s), bulging out at the sides, 

 forms by its farther development the inferior maxilla ; the pos- 

 terior (a), smooth and projecting less, gives rise to a pendulous 

 fold which overlaps the first branchial slit, and which this au- 

 xhqiX compares to the operculum of fishes *. When the branchi-/ 

 al apertures have closed, the neck begins to become much longer 

 and narrower in proportion to the liead and body of the chick. 

 The; part immediately before the opercular covering, or between 

 it and the maxilla infer., seems especially to be expanded in pro- ,. 

 ducing this elongation; while the opercular covering itself, and^ 

 the part in which the posterior branchial apertures were situated, j 

 remain adhering to the fore part of the thorax. At the same » 

 time, the carotid arteries are lengthened out, and the other vas- .,^ 

 cular arches in the fore part of the thorax become straighter, and.,^ 

 assume more nearly the position they afterwards have in the ^ 

 adult bird (Fig. 31, t, ^^, r, *,/?). . . . , j/. 



Wliile these changes take place in the branchial hoops ancj^ 

 vascular arches, the Lungs begin to be formed. The rudiments , 

 of these organs may be perceived as early as on the fourth day. 

 The researches of Rathke -f* have shown that the lungs and air 



.^ From the drawings which Rathke has given of the foetal fish, there can 

 be no doubt of the correctness of this comparison ; but he seems to have,,- 

 omitted to observe, that, before the formation of the opercular body in the . ^ 

 foetus of the chick, the most anterior branchial aperture is closed. 



' ' -'Vi 



•f Sur le Developpement des Organes Respiratoires, in Breschet's Reper-. i 



toire d*Anat. et de Physiol. ; and translated in Edinburgh Medical and Sur- 

 gical Journal, Jati. 1830. "^-^^ '•>'P^M X^ 



