270 THE BIOLOGY OF THE FROG CHAP. 



thj middle line to the base of the skull. In front of the 

 lateral process of the parasphenoid bone it gives off the 

 palatine artery, which courses forward along the roof of 

 the mouth; a little farther forward the cerebral carotid 

 arises, entering the skull in the region of the orbit and sup- 

 plying the brain. The third branch, the ophthalmic, passes 

 forward and supplies the eye and some of the neighboring 

 parts. 



The second branch of the truncus arteriosus, or systemic 

 arch, passes outward and then dorsally around the alimen- 

 tary canal, meeting its fellow of the opposite side to form 

 the dorsal aorta. Near its origin each systemic arch gives 

 off a small laryngeal artery, which supplies the larynx and 

 certain muscles of the hyoid. The small esophageal arter- 

 ies are given off at about the level of the second vertebra ; 

 they are distributed to the dorsal side of the esophagus. 

 The occipito-vertebral artery arises slightly beyond, or some- 

 times in common with, the last and passes forward across 

 the transverse process of the second vertebra ; it then 

 divides into two branches which run above the transverse 

 processes of the vertebrae near the centra. 



The posterior branch, or vertebral artery, extends back- 

 ward along the spinal column. The anterior branch, or 

 occipital artery, runs forward to the head, giving branches 

 to the upper and lower jaw, orbit, and nose. 



The large subclavian artery arises immediately behind the 

 occipito-vertebral, and passes laterally, giving branches to the 

 shoulder and body wall, and then, as the brachial artery, 

 supplying the arm. 



The two systemic arches unite at about the level of the 

 sixth vertebra to form the dorsal aorta, which proceeds back- 

 ward beneath the vertebral column. At the point of meet- 

 ing of the two systemic trunks, but usually more on the left 



