THE FROG 23 
specimen. With fine scissors cut away the pericardium from the 
aortic arches, then with fine forceps carefully pick away the 
muscles ‘and other tissues wherever necessary to expose the 
arteries. If the specimen is a female it may be necessary to re- 
move the ovary and oviduct from one side. If possible leave the 
specimen in such condition that it may be used for the study of 
the arteries by future classes. 
The three subdivisions of the aortic arches are continued on 
each side into the main arterial trunks, the carotid arch, the 
systemic arch, and the pulmo-cutaneous arch. 
1. The carotid arch is the most anterior of the three and is 
formed by the common carotid artery. A short distance 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 superficial muscles 
from the ventral surface of the floor of the mouth. The lingual 
artery often fails to be injected satisfactorily. 
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 internal 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 com- 
ing forward 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 dis- 
secting away the large eye muscle already exposed); and the 
cerebral carotid, which cannot readily be traced since it passes 
directly into the cranium and supplies the brain. 
2. The systemic arch is the middle one of the three main 
arches. It passes dorsalward above the cesophagus and posteri- 
orly, giving off branches on its way, to meet its fellow with which 
