The Arteries 



323 



At approximately the fifth or sixth week of embryonic life, 

 the heart leaves the region of the pharynx and passes backward, 

 while the neck becomes elongated and the head extended for- 

 ward, which causes a great lengthening of the common carotid 

 artery. 





OCL 



Fig. 51. Arrested Development of the Uterus .\nd Vagina. Cow. 



O, Ovary. CL, Corpus hiteum. UC, Uterine cornua. 

 U, Cordiform Mullerian ducts representing the uterus. 

 Va, Vagina. H, Hymen. V, Vulva. MU, Meatus urinarius. 



During the formation of the cranial nerves, the inferior laryn- 

 geal divisions of the pneumogastric nerves cross behind the 

 fourth, or permanent, aortic arches between the aorta and 

 truncus arteriosus, to reach their destination in the muscles of 

 the larynx. As the heart recedes toward the chest and the 

 head grows forward, the laryngeal nerves are necessarily caught 

 and dragged along with the aortse into the chest cavity, thus 

 bringing about their recurrent course. 



While the heart is moving backward, the right aortic, or sys- 

 temic, arch becomes smaller and finally disappears. As a result, 

 the right laryngeal nerve is released from the dragging of the 

 aorta but still passes around the right subclavian artery, while the 

 persistence of the left arch, with its increased size, to become the 

 common aorta continues the dragging upon the left recurrent 

 nerve, which, throughout the life of the animal, must pass into 

 the chest, around the aorta, and retrace its way back to the 

 larynx. This essential difference in the course of the two in- 

 ferior laryngeal nerves is by some presumed to have a funda- 

 mental relation to the fact that, in the affection of " roaring " 

 in horses, it is regularly the left nerve which becomes degenerated, 



