THE LOWER VERTEBRATA. 



103 



of the head is a series of vertical cartilaginous bars, the 

 visceral arches, supporting the walls of the tadpole's gill- 

 slits. The mandibular arch ( = MeckePs cartilage + the 

 palato-pterygoid) is counted as the first of these; the 

 second is called the hyoid arch, and the four following, 

 the first, second, third, and fourth branchial arches. 

 Altogether there are four gill-slits, and between the hyoid 

 arch and the mandibular arch there is an imperforate 

 slit (i.e. a thinning of the sides of the throat from 

 within, which never becomes an actual perforation), which 

 becomes the tympano-Eustachian recess. The frog no 

 longer breathes by gills, but by lungs, and the gills are 

 lost, the gill-slits closed, and the branchial arches conse- 

 quently much reduced. The hyoid arch becomes attached 

 to the otic capsule, and its median ventral 

 plate, including also vestiges of the first, 

 second, third and fourth branchial arches, 

 is called the " hyoid apparatus." In fig. 

 84 the apparatus is seen from the side ; 

 the hyoid arch is now called the "an- 

 terior cornu of the hyoid." The function 

 of the hyoid apparatus in the frog is to 

 furnish a basis of attachment to the 

 tongue muscles and muscles of respira- 

 tion; it remains cartilaginous with two 

 exceptions the relic of one branchial 

 arch, which ossifies as the thyro-hyal ; 

 and (though this Is not altogether certain) the top of the 

 hyoid arch, which is said to become the columella auris. 

 It should be noted that, as development proceeds, the 

 angle of the jaw swings backward, and the hyoid apparatus 

 shifts relatively forward. 



Before pioceeding to the comparison of the mammalian 

 skull with this, we would strongly recommend the student 

 thoroughly to master this portion of the work ; and in no 

 way can he do this more thoroughly and quickly than by 

 taking a parboiled frog, picking off the skin, muscle, and 

 connective tissue from its skull, and making out the various 

 bones and cartilaginous regions with the help of our 

 diagrams. 



Fig. 84. HYOID OF 

 ADULT FROG. 



Tbyro-hyals (posterior 

 corona) black. 



