08 A Manual of Veterinary Physiology. 



articulating heads remains as a fixed point, simply turning on 

 its centre, whilst its fellow describes an arc. This is why the 

 movement can only occur on one side at a time. The same 

 observer, noticing the one-sided mastication in the ox : states 

 that the first stroke of the molars is in an opposite direction 

 to the regular action which follows. Thus, if masticating 

 from right to left the first stroke is made from left to 

 right. 



My own observations on the movements of the articulation 

 of the horse do not agree with Gamgee's. On opening the 

 mouth preparatory to masticating (say from right to left), the 

 right articulation at once rises, the left becomes depressed, 

 and the contents of the orbital fossa on this side bulge : 

 the teeth now crush from right to left, the right articulation 

 descending, the left ascending. As the right teeth meet 

 the right orbital fossa ascends ; by standing in front of the 

 horse we can observe that the elevation of the fossae is 

 alternately performed, that one on the side opposite to the 

 masticating surface bulging the most. 



Id the horse mastication is slow, and as a rule well per- 

 formed ; it takes a horse from five to ten minutes to eat 

 one pound of corn, and fifteen to twenty minutes to eat one 

 pound of hay. In the ox the food does not undergo the 

 same amount of preliminary crushing, as by the process of 

 rumination it is brought back to the mouth for remastica- 

 tion. 



The muscles which close the mouth are the masseter 

 internus and externus, temporalis and pterygoideus ; those 

 which open it are the sterno- and stylo-maxillaris and dia- 

 gastricus. 



The extensive development of the molar teeth in all her- 

 bivora can be readily understood, when we remember the 

 wear and tear to which they are exposed in crushing the 

 fibrous portion of plants. 



The molars wear away more rapidly than the incisors, 

 due to the fact that they perform more work. They become 

 so much reduced in length that they would not meet were 

 it not that the incisors become more oblique. 



