No. i.] MORPHOLOGY OF VERTEBRATE HEAD. 105 



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Vol. IV. 1888. 



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LETTERING. 



II.-VII. Cranial nerves. 



V, an, distal portion of the primary trochlearis. 



1-3, head cavities. 



I*- -, part of the premandibular cavity forming the inferior oblique eye-muscle. 



2 s - °; part of the mandibular cavity forming the superior oblique eye-muscle. 



a, anterior head cavity. 



aa, anterior part of the dorsal aorta. 



aa', anterior part of the dorsal aorta taken into the cardinal system. 



ac, branches of the dorsal aorta. 



ac', branches of the dorsal aorta taken into the cardinal system. 



aud., auditory vesicle. 



b., break in the walls of the mandibular cavity. 



