ADAMS, PHYLOGENY OF THE JAW MUSCLES 135 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Depressor mandibular. 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY V 3 ) 



Capiti-mandibularis. — In Cynognathus this mass of muscle, which is 

 the reptilian muscle of the same name, must have been well subdivided; 

 thus there was probably a large temporalis or medius slip filling the large 

 temporal fossa and a superficial or masseter slip arising along the zygo- 

 matic arch, both outside and inside, both muscles being attached on the 

 large dentary. The large carnivorous jaws of this form indicate a corre- 

 spondingly powerful muscle which not only filled the temporal fossa but 

 also must have been attached to the outside of the zygomatic arch. The 

 attachment must have been on the ascending process of the dentary, for 

 the posterior bones of the jaw are too small. Watson (1912, p. 581) says: 



It is a remarkable fact that in Cynodonts increasing size of the dentary and 

 of the masticatory muscles, which in Cynognathus must be inserted on it, is 

 correlated with a reduction and weakening of the back part of the jaw, which 

 alone articulates with the skull ; not only are the actual bones small, but their 

 attachment to the dentary is weak; they merely rest in the groove in that 

 bone and are often displaced in the fossil skulls. 



As the ascending process of the dentary affords ample space, the capiti- 

 mandibularis must have been attached chiefly to this bone, as in mammals. 



(a) Capiti-mandibularis super ftcialis. — On the outside of the dentary 

 there is a large fossa for the masseter, which must also have been large to 

 match the large temporal mass. It probably had a hold on the inner and 

 outer side of the zygomatic arch, extending forward to the large anterior 

 tubercle on the lower border of the zygoma. The attachment is probably 

 in the depression on the outside of the dentary. From the condition in 

 other reptiles, this superficial slip could extend partly over the rest of the 

 mass and be connected with the fascia of that region. As mentioned 

 before, the pull of these muscles would tend to pull the jaw upwards 

 against the squamosal and not so much against the quadrate. This gave 

 the posterior part of the jaw the opportunity to complete its separation 

 from the dentary in some more advanced form of theriodont and to enter 

 on its new mammalian function in connection with the middle ear. 



(b) Capiti-mandibularis medius. — This part of the temporal mass 

 would fill the temporal fossa and be inserted on the inner and outer sides 

 of the dentary. 



