NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 9 



nent set that is found in the adult, while those that precede and follow 

 (both of which have been demonstrated) disappear.' 



The fact that mastication ceases and that the teeth become stunted 

 has a great influence on the chewing muscles and the jaws. It was 

 necessary for the first whales as fish catchers to be able to open the 

 mouth wide. The masseter muscle which has the tendency to limit 

 the opening of the mouth was therefore little used, and it became 

 restricted ; together with the muscle the region of its origin became 

 shrunken. This region is the anterior and median part of the 

 zygoma ; it is transformed into a slender bridge of bone. The tem- 

 poral muscle has been more used, but it also shows the tendency to 

 be reduced by lack of vigorous use, and it draws itself backward quite 

 low on the side of the braincase, losing its influence on the zygomatic 

 process of the squamosal. This process shrivels up like the coronoid 

 process of the mandible, the muscle's point of insertion. In cases 

 where the under jaw becomes very large the temporal muscle may 

 acquire renewed strength and may spread its region of origin out 

 over its surroundings in an unaccustomed manner. With the atrophy 

 of the teeth they cease to influence the body of the mandible, which 

 consequently loses its original height. The alveoli become less defined 

 and the partition walls between them may disappear so that there 

 arises a common dental furrow. The articular condyle of the 

 mandible weakens, loses its cylindrical form, and the articular sur- 

 face becomes an almost flat area pointing backward at the similarly 

 formed glenoid fossa on the squamosal, which as good as loses its 

 postglenoid process and is otherwise inclined to suffer reduction. It 

 may happen, however, that the lower jaw becomes huge and that its 

 articular condyle acquires corresponding heaviness. In such cases 

 the condyle is curiously modified, losing the true articular surface. 

 This is grown over by articular ligament, and the lower jaw stimu- 

 lates the squamosal to grow out in prodigious size, bearing, instead 

 of the true articular surface, an area of attachment on a projecting 

 foot. The symphysis menti. long in the most primitive whales, is 

 restricted. The under jaw's degeneration is also no doubt indicated 

 by the huge gaping posterior entrance to the mandibular canal, which 

 is mostly filled with loose connective tissue. It is not clear what the 

 reason is for this peculiarity, which was already present in the most 

 primitive cetacea and is found in all the later ones though sometimes 

 in a rather disguised form ; possibly it might in some way depend on 

 the air-sacs of the nasal passages which lie exactly internal to this 

 part of the lower jaw. 



