148 DIGESTION. 



forates the stylo-hyoid, is connected with the hyoid bone by 

 a loop of fibrous tissue ; and acting from this bone as the fixed 

 point, the anterior belly must of necessity tend to depress 

 the jaw. The attachments of the mylo-hyoid and the genio- 

 hyoid render their action in depressing the jaw sufficiently 

 evident, which is also the case with the platysma myoides, 

 acting from its attachments to the upper part of the thorax. 



It has been a disputed question whether the upper jaw 

 does or does not participate in the act of opening the mouth. 

 That depression of the lower jaw is the main action in ordi- 

 nary mastication is sufficiently evident ; but it is possible, 

 by fixing the lower jaw, to perform the acts of mastication 

 laboriously and imperfectly it is true by movements 

 of the upper jaw. In ordinary mastication, the upper jaw 

 undergoes a slight movement of elevation in opening the 

 mouth ; and this becomes somewhat exaggerated when the 

 mouth is opened to the fullest possible extent. Without 

 citing the various authorities for and against this opinion, it 

 will be sufficient, perhaps, to mention the following simple 

 experiment, suggested to Monro by Pringle : " If," says he, 

 " you place the blade of a knife or the finger nail in a situa- 

 tion which corresponds precisely with the point of contact of 

 the teeth, when the mouth is closed, the knife being held in 

 a fixed position during the time when the mouth is opened, 

 it can be observed in a mirror that the upper teeth are sensi- 

 bly elevated every time the mouth is opened." 1 



Many speculations have been put forward by those who 

 adopt this view, as to the precise muscular action involved 

 in this movement, which is necessarily a movement of the 

 entire head. It is possible that the posterior belly of the 

 digastric may have such an action, to a slight extent. The 

 movement of the head, however, does not ordinarily require 

 any powerful muscular action, and is probably the result of 

 the contraction of muscles too deeply situated to be ex- 

 plored experimentally. It is evidently not due, as a rule, to 



1 Cited by BERARD, Cours de Physiologic, Paris, 1848, tome i., p. 617. 



