940 



OF GENERATION EMBRYONIC DEVELOPMENT. 



as the Ductus Arteriosus. 1 A knowledge of these different stages in the de- 

 velopment of the Heart and Arterial system enables us to explain many of 

 the malformations which they occasionally present in Man ; these being for 

 the most part due to arrest of development, whereby the circulating appa- 

 ratus is permanently fixed in conditions that are properly characteristic of 

 cold-blooded animals. And it is interesting to remark, too, that the varie- 

 ties which not uufrequently present themselves in the arrangement of the 



c' 



ad 



\. Truncus arteriosus, with one pair of aortic arches, and dotted outlines indicating the future posi- 

 tion of the second and third pairs. 2. Truncus arteriosus, with four pairs of aortic arches and indica- 

 tions of the fifth. 3. Truncus arteriosus, with the three posterior pairs of aortic arches, from which 

 the permanent vessels of the embryo are developed, w.ith dotted outlines showing the position of the 

 two (now) obliterated anterior arches. 4. Permanent arterial trunks in their primitive form, the ob- 

 literated portions still shown in dotted outline. 1-5. Primitive aortic arches; a, aorta; p, pulmonary 

 artriy; p',p", branches to the lunss; aw', root of thoracic aorta (ad) on left *ide ; aw, obliterated root 

 springing from right side; s', s", subclavian artery; v, vertebral; a.r, axillary; c, common carotid; c', 

 external carotid ; c", internal carotid. In the chick the part s' has been observed to atrophy, the 

 vessel c becoming the external, and c' the internal carotid artery. 

 . 



principal trunks given off from the Aorta, find their analogues in the 

 arrangements that are normally characteristic of some one or other of the 

 Mammalia. 



782. The Venous system of the body generally undergoes changes which 

 are even more remarkable than those of the arterial trunks. In its earliest 

 condition, it has been ascertained by Rathke 2 to present essentially the same 

 type in the embryos of all Vertebrated animals; the peculiarities of each 

 group being acquired by a process of subsequent transformation. There is 

 at first a pair of anterior venous trunks (Figs. 344, 345, y, (/}, receiving 

 the blood from the head, and a pair of posterior trunks (/, /' ), formed by 

 the confluence of the veins of the trunk, of the Wolffiau bodies, etc. ; the 

 former are persistent as the jugular veins; the latter remain separate- in 

 most Fishes, where they are designated the cardinal veins; but in Man (as 

 in warm-blooded Vertebrata generally), they are only represented by the 

 venve azygos, major and minor, 3 which coalesce into a common trunk for a 

 considerable part of their length. One of the anterior trunks and one of 

 the posterior unite on either side, to form a canal which is known as the 

 Ductus Cuvieri ; and the ducts of the two sides coalesce to form a shorter 

 main canal, which enters the auricle, at that time an undivided cavity. 

 This common canal is absorbed into the auricle at an early period, in all 

 Vertebrata above Fishes; and after the septum auriculoium is formed, the 

 two Cuvierian ducts separately enter the right auricle. This arrangement 

 is persistent in Birds and the inferior Mammals, in which we find two VenaB 



: Kollikor, Entwickelungsgeschichte des Monschen, p. 409. 



2 Uchcr den Ban und die, Entwickelung di-s Venensystems tlrr Wirbeltbiere, 1838. 



3 See Mailer's Vergleieliemle Anatomic der Myxinoiden, Berlin, 1840. 



