DIGESTION 157 



produce, and necessitates as a rule about 40 crushings between 

 the molars before being fit for swallowing. This data gives some 

 notion of the amount of work performed by the masseter muscles 

 in eating, say, 2 kilogrammes (4*4 pounds) of hay. Colin, whose 

 results are given above, shows that it took 5 horses, the following 

 time to eat 2 kilogrammes of hay : 1 hour, 1 j- hours, 1 hour 

 12 minutes, ij hours, if- hours, the latter being a very small 

 horse, while the second animal in the series was a big one. The 

 average rate of crushing is 70 to 80 per minute, while the amount 

 of work performed by the jaws working at 420 to 480 times an 

 hour constitutes a distinct source of daily loss. Colin shows 

 that when the animal is very hungry he will prepare and swallow 

 30 balls in 15 minutes, but as he gets satisfied he does not make 

 more than 10 or 12 swallows in the same time. If the flow 

 of saliva be reduced in amount, the length of time occupied by 

 mastication is naturally increased. 



According to the writer's observation, it takes a horse 15 to 

 20 minutes to eat 1 pound of hay, and 5 to 10 minutes to eat 

 1 pound of corn. 



With the ox the first mastication is imperfectly performed, 

 and is three times quicker than in the horse. When, however, 

 the material is brought back for remastication, the process is 

 slow. In the dog mastication is imperfectly performed ; after 

 a few hasty snaps of the jaw the material is swallowed. 



Opening the mouth is equivalent to depressing the lower 

 jaw, for the upper takes no share in the process. The muscles 

 which open the mouth are comparatively small, for very little 

 effort is required ; the stemo- and stylo-maxillaris and digastricus 

 perform this function. On the other hand, the closing of the 

 jaws in mastication is a difficult task, and for this purpose very 

 powerful muscles exist, they are the masseters, temporals, and 

 pterygoids. In the dog the temporal muscles are considerably 

 developed, whilst in herbivora the masseters are the largest. 



The nerves employed in mastication are the sensory fibres 

 of the fifth which convey to the brain the impulses resulting from 

 the presence of food in the mouth, while the motor fibres of the 

 same nerve supply the needful stimulus to all the muscles of 

 mastication excepting the digastricus, which receives its motor 

 supply from the seventh pair. 



Deglutition. 



The process of swallowing is usually described as occurring in 

 three stages. The first stage practically comprises carrying the 

 food back to the base of the tongue and pressing it against the 

 soft palate ; it is a simple process and readily understood. In 



