156 



PHYSIOLOGY 



CHAP. 



muscular sense of the muscles that come into play ; it is activated 

 by the motor roots of the third branch of the trigeminals, aided by 

 the hypoglossal and facial nerves. The immediate common centre 

 of the masticatory movements, according to Schroder van der 

 Kolk, lies in the medulla oblongata; this view is not, however, 

 supported by any definite anatomical evidence. Since the move- 

 ment is complex and voluntary, its centre probably lies in the so- 

 called motor zone of the cerebral cortex. In fact, the electrical 

 excitation of a circumscribed area in the lower lateral part of the 

 cortex of the anterior lobe of the brain in rabbits readily produces 



FIG. 52. A, internal pterygoid muscles viewed from outside. (G. D. Thane.) The masseter 

 muscle, greater part of zygomatic arch, temporal muscle with coronoid process, and a large 

 part of the ramus of the jaw have been removed ; 1, external pterygoid, the figure is placed 

 on the lower head ; 2, internal pterygoid. B, lower part of skull and face to show attach- 

 ment from behind of pterygoids and some other muscles. (Bourgery.) a, body of spheroid, 

 beneath which are the posterior nares ; b, section through temporal bone ; c, hard palate ; 

 <I, posterior part of condyle and neck of lower jaw, above which are the synovial cavities of 

 the joint separated by the inter-articular nbro-cartilage ; e, symphysis menti ; 1, left internal 

 pterygoid muscle ; 1', lower part of same muscle, on right side, the middle is cut away to 

 show external pterygoid ; 2, lower head of external pterygoid ; 2', upper head of the muscle, 

 attached in part to the inter-articular disc ; 3, origin of mylo-hyoid and genio-glossus muscles 

 from the mental spines ; 4, origin of mylo-hyoid ; 5, attachment of anterior belly of digastric ; 

 6, 6, masseter muscles. 



movements resembling those of mastication. The centrifugal 

 paths from this area lead through the corona radiata to the median 

 segment of the internal capsule, and may be followed into the 

 anterior mesial part of the cerebral peduncle. 



Simultaneously with mastication comes the insalivation of the 

 food, by which its particles are worked into a mass, which, when 

 carried to the back of the tongue, becomes rounded and is called 

 the bolus. The movements of the tongue, besides shifting the 

 food to and fro between the rows of teeth, help in shaping the 

 bolus from the fragments already chewed and insalivated. 



According to Gaudenz (1901) a mouthful suited for mastication 

 normally has a volume of about 5 c.c. Its weight depends on the 

 specific gravity of the foods. Such a mouthful in a normal man 



