TEE FCETUS. 



1043 



The arteries are developed partly at the expense of the vessels 

 primary circulation, and partly in the vascular lamina of the 



Fig. 578. 



of the 

 mbryo. 



The heart, when it was only a simple 



cylindrical tube, presented at its 



anterior extremity two aortic arches, 



which curved backwards and united 



to form the single aorta, then the 



vertebral or common aortte. The 



aortic arches are situated at the inner 



face of the two first pharyngeal 



arches ; afterwards more are de- 

 veloped, which are placed within the 



other pairs of arches, until the num- 

 ber is increased to five, though they 



never all exist at the same time. 



Some atrophy, while others are being 



developed : the two first entirely 



disappear ; the third form the caro- 

 tids ; the fourth the axillary arteries 



and the arch of the aorta ; the fifth 



atrophies on the right, and on the 



left originates the pulmonary artery, 



the ductus arteriosus, and the aorta. 



The latter is continued along the 



spine by the fusion of the two 



primitive aortae. They present, at 



their posterior extremity, the pelvic 



vessels — which are very small — and 



the umbilical arteries, which are, on the contrary, remarkable for their volume 



HEART OF AN EQUINE FCETUS. . THE RIGHT AURICLE 

 AND PUbXERIOR VKNA CAVaWaVE BEKN OPENED 

 TO SHOW THE FORAMEN OF BOTAL. 



1, Left ventricle ; 2, right ventricle ; 3, interior of 

 the right auricle ; 4, posterior vena cava ; 5, 

 foramen of Botal. 



PLAN OF THE AORTA AND ITS ARCHES AT AN EARLY PERIOD. 



1 Truncus arteriosus, with one pair of aortic arches, and dotted lines indicating the position of the 

 ' second and third pairs ; 2, the same, with four pairs of aortic arches, and indications of the fifth ; 

 3, the same, with the three posterior pairs of aortic arches, from which the permanent vessels of 

 the embryo are developed, with dotted outlines showing the position of the two (now) obliterated 

 anterior arches ; 4, permanent arterial trunks in their primitive form, the obliterated portions 

 still shown in dotted outline, 1-5, primitive aortic arches, a. Aorta ; p, pulmonary artery ; 

 p', p", branches to the lungs ; aio', root of thoracic aorta (ad) on leit side ; aw, obliterated root 

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

 carotid ; c', external carotid ; c", internal carotid. 



The peripheral arteries arise, independently of the central vessels, on the 

 interior of the vascular lamina. They appear in the form of solid cellular 



