156 A MANUAL OF VETERINARY PHYSIOLOGY 



simple, being only a depression and elevation of the jaw ; this 

 motion means a simple temporo-maxillary articulation, and such 

 is met with in this animal. In the horse and ox the movement 

 is not only up and down, but lateral, and some say even from 

 front to rear. This necessitates a complex joint capable of 

 affording a considerable amount of play, and this is provided 

 by a disc of cartilage being placed between the articulation, 

 which accommodates -itself to the varying movements of the 

 joint in the horse, ox, and sheep, and also saves the part from 

 jar. In herbivora, therefore, we find the cartilage extensively 

 developed, whilst in carnivora it is small and simple. The 

 character of the movement occurring in the temporo-maxillary 

 articulation of herbivora during mastication is as follows : During 

 rotatory movement, or lateral displacement, one of the articu- 

 lating 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 (Colin). During mastica- 

 tion the contents of the orbital fossae are observed in the horse 

 to be alternately ascending and descending. This movement is 

 due to the coronoid process of the lower jaw, the fossa being 

 pushed up as the process comes forward and depressed as it 

 recedes. The muscles which bring about this important lateral 

 movement of the jaws, which in the ox, owing to the freedom 

 of the articulation, may be termed rotatory, are the two ptery- 

 goids, especially the internal. The herbivora can only masticate 

 on one side at a time ; when tired on one side the process is re- 

 versed, and the opposite molars take on the crushing. It is 

 surprising the length of time an animal will carry on mastication 

 on one side ; even as long as an hour has been observed in the 

 horse by Colin. This observer noticed that in the ox the first 

 stroke of the molars is in the opposite direction to the regular 

 action which follows ; thus, if masticating from right to left the 

 first stroke is made from left to right. It is important to note 

 that in those animals where a single-sided lateral or rotatory 

 movement in mastication is necessary, the upper jaw is always 

 wider than the lower ; this we can understand, for if both were 

 the same width the molar teeth would not meet each other 

 when the jaws were crossed for lateral mastication. This extra 

 width of the upper over the lower jaw, in conjunction with the 

 peculiarity of mastication, explains why the molar teeth of the 

 horse and other herbivora wear with sharp chisel edges (see 



Fig- 57)/ 



Mastication in the horse is a slow process, though ' greedy ' 

 feeders are not unknown. The grinding is very thoroughly 

 performed. The resulting semi-liquid mass weighs from 50 to 

 100 grammes (1 to 2 ounces), takes about half a minute to 



