ORIGIN OF JAW APPARATUS aie 
front edge also serves as the fulcrum over which the power- 
ful pulls are exerted in the rasping and tearing action of the 
teeth of the jaw. It further serves as the firm base of attach- 
ment of a large group of muscles which take part in the work of 
operating the jaw and in closing the buccal cavity as a whole. 
The attachment of the distal ends of the jaw bars to the anterior 
end of the base plate or hyoidean apparatus is well accounted 
for by the close association of the two structures both in posi- 
tion and in functioning and by the necessity for the separation 
of the distal from the proximal division of the jaw bar in order 
to secure freedom of motion of the latter. The position of the 
distal part of the jaw bar varies with the degree of contrac- 
Fig. 24 The adult cartilaginous skeleton of the jaw apparatus of the right 
side, including the corneal cartilage, anterior end of jaw and hyoid to which the 
jaw bar attaches. Bdellostoma. 
- tion of the snout region of the fish, but, as figures 22, 23, 24, 
25, 26, 29 show, it runs forward and upward at an angle that 
recalls the slope of the jaw bars in Amphioxus. 
The hyoidean apparatus has acquired cartilaginous connec- 
tion with the skull above (figs. 25, 26, 27), and is also connected 
thereto by strong tendinous bands. ‘The jaw itself has no attach- 
ment to the cranial skeleton except the tendinous attachment to 
the anterior end of the nasal capsule and a similar connection 
through the corneal cartilage with the palatine and subnasal 
bars. As the jaw works longitudinally while in the head, it 
has no vertical stresses not already provided for by the hyoidean 
supporting mechanism, and it is the latter which has developed 
the cranial attachments mentioned above. 
