320 Veterinary Obstetrics 



after birth, giving rise to the affection known as persistent fora- 

 men ovale, or cyanosis of the newborn, by which the blood re- 

 mains mixed, as in the serpent, giving a bluish color to the skin 

 and mucous membranes, usually ending in the early death of the 

 young animal. 



The constriction between the auricular and ventricular por- 

 tions gradually develops to form the septum between these cavi- 

 ties and, from these also, the auriculo- ventricular valves are 

 formed. 



The ventricular cavity becomes divided into two somewhat 

 unequal halves toward the close of the first month. There also 

 forms in the trimciis arteriosus, ta, Fig. 49, a median partition, 

 which, growing backwards, divides the vessel into two parts, 

 which are to represent in the future the common aorta and the 

 pulmonary arter}^ The septum between the two ventricles is 

 for a time incomplete and both the aorta and pulmonary artery 

 are connected with the right ventricle only, so that the left 

 cavity communicates with the arteries only through the opening 

 in the ventricular septum. 



The ArteriEvS. 



A series of aortic arches, see between va and da, Fig. 49, 

 ari.se early in the third week of embryonic life from the anterior 

 extremit}^ of the truncus arteriosus, one for each of the visceral 

 arches, the more anterior appearing first. At first there are two 

 wholly separate aortae, which run parallel to each other through- 

 out the length of the body just beneath the notochord, but they 

 soon become fused po.steriorly to constitute the dorsal aorta, 

 from which is given off the vitelline arteries to the vitelline sac. 

 Further back it divides into the two allantoic arteries, which 

 carry the blood to the allantois. The aortic arches pass from the 

 floor of the pharynx up through the visceral arches on either 

 side and open into the aorta. They are soon complete, five, or^ 

 according to some authors, six pairs, being present, one for each, 

 branchial arch, and all connected above with the dorsal aorta. 



The attachment of the truncus arteriosus begins to shift back- 

 ward along the floor of the mouth and divides into two branches, 

 the anterior of which maintains connection with the mandibular 

 and hyoidean aortic arches, w^hile the posterior is connected with 

 the three, or four, remaining vessels. 



