430 GRACE A. WELLS 



general structure of these arches is best shown in a ventral view 

 (fig. 6) the arches themselves being laid open as much as possible 

 and still retain their connections. 



The most anterior arch, the mandibular ( md.) is the largest 

 and the most modified (see also fig. 9). It is divided into a dor- 

 sal and a ventral portion, these articulating with each other 

 posteriorly, and forming the upper and lower jaws. The upper 

 is the pterygoquadrate (palatoquadrate), the lower is Meckel's 

 cartilage. Each is composed of right and left halves which are 

 articulated in the middle line. 



The pterygoquadrate is composed of two curved and verti- 

 cally flattened bars, thinnest anteriorly where they meet in the 

 middle line. At about the anterior third each bears a strong 

 palatine process on its upper border (p.p.) which plays against 

 the palato-basal articular surface of the cranium. From this 

 point the bar narrows posteriorly and then expands into the 

 quadrate portion bearing above an expansion, concave ex- 

 ternally for the attachment of the adductor mandibularis mus- 

 cle. From this an articular process extends downwards and 

 backwards and bears on its distal extremity a convex surface 

 with which Meckel's cartilage articulates. On its anterior sur- 

 face the pterygoquadrate is convex from above downwards; on 

 the posterior or oral side the anterior two-thirds presents a 

 marked alveolar groove, in which the functional and reserve 

 teeth are situated. The ventral margin of the groove is sharp. 



The Meckelian cartilage is thin, curved in the horizontal 

 plane, and gradually increasing in height from the symphysis 

 to near the posterior angle. On the oral side there is an alveolar 

 groove, similar to and of about the same extent as that in the 

 upper jaw. The inferior border of this groove extends obliquely 

 upwards and backwards and is continued into a strong coro- 

 noid process which connects distally with the lower anterior 

 angle of the hyomandibular cartilage. Just behind this process 

 the lower part of the Meckelian curves strongly outwards into 

 an angular process, on the upper surface of which is a concave 

 facet for articulation with the pterygoquadrate. Below this 

 the process ends in a right angle. On the outer side of the jaw 



