328 , HUMAN PHYSIOLOGY 



One of these plexuses (Auerbach's) is placed between the 

 muscular coats the other (Meissner's) is placed in the 

 submucosa. 



II. Physiology. 

 I. Digestion in the Mouth. 



A. Mastication. In the inouth, by the act of chewing, 

 the food is thoroughly broken up and mixed with saliva. 



The muscular mechanism of mastication may be here 

 briefly indicated. 



MOVEMENTS OF MASTICATION. Movements of Lower Jaw 

 in 1. Vertical Plane, a. Elevation, a. Temporal. @. 

 Masseter. 7. Internal Pterygoid. b. Depression, a. Weight 

 of Jaw. $. Anterior Belly of Digastric. 7. Mylo- and 

 Genio-Hyoid. B. Platysma. Hyoid fixed by Omo- and 

 Sternohyoid, Sternothyroid, and Thyrohyoid. 2. Horizontal 

 Plane Forwards: External Pterygoids. Backwards: Pos- 

 terior Fibres of Temporal. To Right: Left External Ptery- 

 goid. Right Temporal (Posterior Fibres). Other Muscles of 

 Mastication Buccinator and Orbicularis oris. 



B. Saliva. The saliva is formed by the salivary glands 

 (viz., the parotid, submaxillary, sublingual, and various 

 small glands in the mucous membrane of the mouth). 



Characters. It is a somewhat turbid frothy fluid which, 

 when allowed to stand, throws down a white deposit consist- 

 ing of shed epithelial scales from the mouth, leucocytes, 

 amorphous phosphates of lime and magnesia, and generally 

 numerous bacteria. Its specific gravity is low generally 

 about 1003. In reaction it is neutral or faintly alkaline. 



Chemically it is found to contain a very small proportion 

 of solids. The saliva from the parotid gland contains only 

 about 0'4 per cent., while that from the sublingual may 

 contain from 2 to 3 per cent. The sublingual and sub- 

 maxillary saliva in man is viscous, from the presence of mucin 

 formed in these glands, while the parotid saliva is free from 

 mucin. In addition to mucin, traces of proteids are readily 

 demonstrated, and with these proteids is associated the active 

 constituent or enzyme of the saliva ptyalin. 



Saliva generally contains traces of sulphocyanide of potas- 



