TUE F(ETV8. 



1043 



The arteries are developed partly at the expense of the vessels of the 

 primary circulation, and partly in tlie vascular lamina of the embryo. 

 The heart, when it was only a simple 



cylindrical tube, presented at its • ■ • 



anterior extremity two aortic arciies, 

 which curved backwards and united 

 to form the single aorta, then the 

 vertebral or common aortte. The 

 aortic arches ai'e situated at the inner 

 face of the two lirst 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 tiiey 

 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 POSTKRIUR VENA CAVA HAVE BEKN OPENED 

 TO SHOW THE FOKAMEN OF BOTAL. 



1, Left ventricle ; 2. right ventricle ; H, interior of 

 the right auricle; 4, posterioi- vena cava; 5, 

 foramen of Botal. 



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



1 Truncus arteriosus, with one pair uf aortic arch s. and dotted lines indicatini; the position of the 

 second and third jiairs ; 2, the :^ame, 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 ; aw', root of thoracic aorta (ad) on left side ; aw, obliterated root 

 springing from right side; s". s', subclavian artery; v, vcrtelrral ; ax, a.xiliary; c, common 

 carotid ; c', e.xternal 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 



