THE FROG 35 



from its origin it divides into the small external carotid or 

 lingual artery and the internal carotid artery. 



The lingual artery passes forward, giving off branches 

 to the thyroid gland, various muscles of the hyoid apparatus, 

 and the tongue. Trace it after removing the skin and super- 

 ficial muscles from the ventral surface of the floor of the 

 mouth. The lingual artery often fails to be injected satis- 

 factorily. 



The internal carotid artery. Just beyond the lingual 

 artery the internal carotid enters a small swelling of spongy 

 structure known as the carotid gland. This organ, which 

 often blocks the injection, is pigmented; beyond it the in- 

 ternal carotid turns first dorsalward, then anteriorly above 

 the skin of the roof of the mouth to the region of the eye. 

 To trace its distribution in this region pry open the mouth 

 of the frog and make a longitudinal slit in the median line 

 through the skin of the roof of the mouth. Turn back the 

 flaps and observe the internal carotid artery coming for- 

 ward to the region of the eye. Here it breaks up into a 

 number of smaller arteries, the most important of which 

 are the palatine, which courses forward along the roof of 

 the mouth ; the ophthalmic, which passes directly to the eye 

 (find it by dissecting away the large eye muscle already ex- 

 posed) ; and the cerebral carotid, which cannot readily be 

 traced since it passes directly into the cranium and supplies 

 the brain. 



2. The systemic arch, or systemic artery, is the middle 

 one of the three aortic arches. It passes dorsalward above 

 the oesophagus and posteriorly, giving off branches on its 

 way, to meet its fellow from the opposite side, with which 

 it unites in the median line to form the dorsal aorta. The 



