ADAMS, PHYLOGENY OF THE JAW MUSCLES 79 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Adductor hyomandibularis (Ad. h.). 



Adductor braiichialis (Pollard). 



The differences in the musculature of the shark and of Polypterus are 

 due to the fact that the latter has a bony skull roof and requires a differ- 

 ent form of musculature. 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY V^) 



Adductor mandihulce. — This muscle is somewhat more complex than 

 the adductor of the selachians, where the mass is rather compact. It 

 may be divided into a temporal (Adm-), masseter (Adm^) and pterygoid 

 portions (Adm^). 



(a) The masseter (Adm^) is the largest part of the adductor and 

 stretches across the face of the mass, so that the pull is almost along the 

 line of the skull base. It is covered by the preopercular plate, so that it, as 

 well as the rest of the adductor, is not seen from the outside. The muscle 

 slip arises along the back part of the skull where the preoperculum joins 

 the hyomandilnilar and quadrate. These bones bound a triangular region 

 at the posterior part of the skull where the masseter slip arises. It is 

 attached to the preoperculum along the inner face, where it overlaps the 

 hyomandibular and extends to the quadrate. The insertion is on the coro- 

 noid process of the mandible. 



(&) The temporal portion (Adm^) of the adductor, as well as the 

 pterygoid slip, are at right angles to the masseter portion, so that the 

 combined pull of the three parts is at an angle of about 45° to the top of 

 the skull as a base line. This slip arises on several bones of the dorsal 

 part of the skull, on the postorbital and frontal. The attachment to the 

 frontal (after Pollard, 1892, p. 391) is "to its under surface between its 

 projecting edge and its articulation with the orbito-sphenoid, extending 

 even above the eye." The insertion of the temporal portion is in the 

 suprameckelian fossa. This part is anterior to the pterygoid slip and 

 overlies it somewhat. It is at right angles to the masseter. 



(c) The pterygoid slip (Adm^) arises behind the temporal and be- 

 neath it, on the orbitosphenoid and parasphenoid, and its insertion is on 

 the mandible along with the temporal. This mass of muscle represents 

 a part of the capiti-mandibularis of reptiles and the adductor of the 

 selachians. 



Protracior liijomandihularis (P. h.). — Two muscles connected with the 

 adductor group assist in the movement of the operculum, the protractor 



