41() THE EABBIT. 



The truncus arteriosus becomes divided, much as in the chick 

 or in the frog, by an internal longitudinal septum ; which, arising 

 between the roots of the systemic and pulmonary arches, grows 

 backwards in a somewhat spiral course, dividing the truacus 

 arteriosus into right or pulmonary, and left or aortic tubes. 

 The septum continues its growth backwards until it meets the 

 upper free edge of the ventricular septum, with which it fuses. 



After the truncus arteriosus is thus divided internally, an 

 external groove appears on its surface, opposite the internal 

 septum ; and this groove deepens until it splits the truncus 

 arteriosus into two completely separate and independent ves- 

 sels, of which the right one, or pulmonary trunk, arises from the 

 right ventricle, and the left one, or aortic trunk, from the left 

 ventricle. 



The semilunar valves are formed by projections of the 

 thickened endocardium at the roots of the pulmonary and aortic 

 trunks : the valves are at first thick and soft, but later on 

 become membranous. 



2. The Arteries. 



In the rabbit, as in the chick, five pairs of aortic arches are 

 developed, which appear in order from before backwards. By 

 the middle of the tenth day the first two pairs are present, in 

 the mandibular and hyoidean arches respectively. By the end 

 of the tenth day a third pair of aortic arches is present, in the 

 first branchial arches ; and before the end of the eleventh day the 

 remaining two pairs are established, in the second and third 

 branchial arches respectively. 



Of these five pairs of aortic arches, the first two pairs, in the 

 mandibular and hyoidean arches respectively, lose their connec- 

 tions with the dorsal aortee during the eleventh day, and become 

 reduced to the arteries of the floor of the mouth and of the 

 tongue. 



The aortic arches of the third pair, in the first branchial arches, 

 persist as the caxotid arteries. They retain for a time their 

 connections at their dorsal ends with the fourth pair of arches, 

 but ultimately lose these, and from this time send blood to the 

 head alone ; each divides into external and internal carotid 

 arteries, supplying the parts outside and inside the skull 

 respectively. 



