30 DR. E. LOXjSTBERG OX DIGESTIVE [Jan. 14, 



whereby the incisors become sepai'ated. In the Phalangerids in 

 question I think that in a similar manner as in the Rodents the 

 masseter may press the mandibular incisors together, but the 

 pterygoideus intermcs separates them from each other. The 7n7/lo- 

 hyoideus seems to be rather weak, and is inserted too high up on 

 the inner surface of the mandible to have any power of bending 

 the lower parts of the mandibular rami inwards and thus 

 separate the incisors. The pterygoideus internus is very strongly 

 developed, and the angle of the mandible is much inflected so as 

 to give this muscle a wide area of insertion. It is of interest 

 to see that in those Phalangerids which have especially movable 

 mandibular rami, viz. Pseudochims, Fetaunos, and Acrohates, 

 this inflexion is much stronger than in the Koala with fixed 

 mandibular rami. This fact gains importance by the obser- 

 vation that in the three foi-mer less is done for the enlarging of 

 the area of insertion of the masseter on the outer side of the 

 mandible than in the Koala, in which the angle is considerably 

 expanded on the outer side. 



In Trichosurus and the Wombat, with immovable mandibular 

 rami, and in Phalanger, with the same only a little movable, the 

 angle of the mandible is broadly expanded on both sides for 

 the purpose of giving the powerful muscles a wide area of 

 insertion. 



It has been stated above that the Koala and Pseiidochirus, and 

 in a somewhat smaller degree Trichosurus and Phalanger, must 

 move their mandibles in a lateral dii"ection in the act of chewing, 

 so that the mandibular molars come quite opposite those of the 

 maxillary. This movement must take place in such a manner 

 that the whole mandible makes a slight lateral rotation with the 

 condyle of the same side, towards which the movement is directed, 

 as a fixed point or j^ivot. This rotation is efiected by the con- 

 traction of a portion of the masseter of the opposite side, that 

 portion which, posteriorly, is inserted to the outer angle of the 

 mandible, and anteriorly to the foremost jDart of the zygomatic 

 arch or to the maxillary below and in front of the same. 



When contracting, this muscle endeavours to pull its posterior 

 point of insertion forwards, which results in a pressing of the 

 whole mandible towards the other side. The result is the more 

 easily obtained the more the outer angle of the mandible is 

 developed, because the lever becomes longer when the posterior 

 point of insei'tion of the said masseteric portion is situated more 

 laterally — or, which is the same, at a greater distance from the 

 pivot (condyle). The m. pterygoideus internus when contracting 

 endeavours to draw up the lower margin of the mandible, or its 

 inner angle, in an upward and somewhat median direction. This 

 action then results in an outward bending of the molar series, 

 and, perhaps, also a slight lateral rotation of the whole mandible, 

 because it is fixed posteriorly. The more the inner mandibular 

 angle protrudes in a median direction, the longer is the lever for 



