in ike Fcetus of Vertebrated Animals. 267 



reversed ; the right innominata taking its origin before the vessels 

 of the left side *. 



' 'The pulmonary vessels appear to be given off by the fourth 

 arch on the right and the fifth on the left side (//^), the fifth 

 on the right being wholly obliterated. While, however, the caro* 

 tid and branchial arteries become developed from the anterior 

 arches, the pulmonary arches do not continue to carry blood to 

 the root of the aorta, as takes place in those of the bird. The 

 parts by which these arches communicate with the root of the 

 descending aorta (forming in birds the ductus botalli) become 

 gradually obliterated, so that of all the five pairs of vascular 

 arches in the embryo of the mammiferous animal, only one, the 

 fourth of the left side, remains prominent. 



While these changes take place in the pulmonary arches, tlie 

 bulb of the aorta, from the single cavity of which the pulmona- 

 ry and systemic vessels arise for some time in common, is di- 

 vided, so as to form the roots of the aorta proper and pulmo- 

 nary arteries. According to Meckel t, the septum which has se- 

 parated the left ventricle entirely from the right, appears to be 

 continued onwards into the bulb of the aorta, and thus separates 

 this cavity longitudinally into two compartments. The divi- 

 sion of the bulb is, however, imperfect for a time ; it advances 

 gradually from the part next the ventricle to that from which 

 the vascular arches rise ; so that, while the posterior part is di- 

 vided, the anterior yet remains single, a communication being 

 left at this part between the aortic and pulmonary roots, which 

 admits of the passage of the ffood from the right ventricle 

 into the aorta, when the pulmonary arches are obliterated 

 (Fig. 39. A). When the division of the aortic bulb has just ta- 

 ken place, the arch and descending part of the aorta appear to 

 be a continuation of the pulmonary rather than of the aortic 

 root, the latter appearing to lead only into the vessels of the 

 head and anterior extremities. The ductus arteriosus remains 

 for some time, as at first, short and wide, and has the appear- 

 ance of being an opening of communication between, or a defi- 

 ciency in, the parietes of the juxtaposed tubes ; it afterwards be- 

 comes lengthened out and narrowed, and appears during a short 

 " In birds, the left innominata comes off from the aorta first 

 t Meckel's Archiv. B. ii. h. 3 ; and Journal Complementaire, torn L 



s2 



