115 



But in connection with this mobility there is quite a different matter that 

 must now be pointed out. 



On examining the flutings or furrows (plicae) running through the layer 

 of blubber on the throat and ventral side, it will strike the observer that they 

 occur precisely in those places beneath which there are no bones but cavities; 

 these cavities can be compressed thus diminishing the volume of the body 

 through the increased pressure of the water when the animal dives to a 

 great depth. 



This diminution of the volume is m the nature of things attended by 

 an increase of the creature's specific gravity thereby of course facilitating the 

 diving. The same purpose is also served by the contraction of the tongue, 

 by which the voluminous mass of the throat is raised and the flutings of 

 the throat are narrowed and closed. 



In both these cases we therefore see a diminution of the volume of the 

 body as a factor of some value in diving, and there are moreover grounds 

 for surmising that a third cause of a diminution of volume is found in the 

 mobility of the lower jaw through contraction of the musculus temporalis. 



The temporalis is a very big muscle completely filling up the temporal 

 fossa and even causing a slight swelling of the flesh. The converging fascicles 

 of the muscle follow the back of the protruding part of the orbit formed 

 by the os frontale, in a downward and forward direction, and terminate in 

 a very strong tendon, which according to Carte and Macalister (3) p. 222 

 attaches itself to the processus coronoideus. (This latter detail appears to me 

 slightly incorrect: the attachment is probably effected at the basal part of the 

 processus coronoideus postero-interioly, where the bone is clearly very rough 

 for a powerful attachment of a tendon. Besides, if the processus coronoideus 

 served for this attachment its rear-edge would of course be indistinguishable 

 from the outside). 



The sketch reproduces the musculus temporalis at e, from which it may be 

 gathered how according as the muscle contracts the inferior maxilla is first 

 drawn backwards, whilst the posterior end of the jaw- bone will also move 

 towards the interior. The diminution of volume conducive to ease in diving 

 can therefore, be it only to a slight extent, be emphasised by the movement 

 of the lower jaw. 



Let us now consider the question whether the lower jaw can be with- 

 drawn so far that the processus coronoideus enters the temporal fossa, i.e. be- 



