314 ALIMENTARY CANAL 



mass to convenient units and for absorbing the same, and 

 posteriorly, a semi-voluntary mechanism for ejection of waste. 



I. The mouth is the port of the alimentary transport system. 

 First, by nose and eye the cargo is sighted and its nature estimated. 

 Messages are sent inland, factories get busy and all is ready when 

 the ship reaches port. By means of the taste buds on the tongue, 

 the nature of the cargo is further ascertained and appropriate 

 secretions from the salivary glands take place. Bitter or saline 

 substances provoke a profuse secretion of watery saliva. Flesh 

 is met by a secretion containing a large proportion of the lubri- 

 cating material mucin. Dry matter causes the flow of a thinner 

 and more watery saliva than moist matter. 



(a) The functions of saliva in the mouth are purely mechanical. 

 It acts as a lubricant : moistening the surfaces of the mouth and 

 the passage from it : infiltering the food mass and so necessitating 

 the expenditure of less energy in milling the food ; and finally 

 covering the outside of the bolus with mucin, thus rendering 

 deglutition easy. Normally, saliva has no chemical action in the 

 mouth. It contains a diastatic enzyme, ptyalin, which, however, 

 carries out its action on polysaccharides during the earlier period 

 of gastric digestion (q.v.). 



(b) The tongue is a mobile organ lying on the floor of the mouth. 

 It consists mainly of a mass of muscles which are paired. Some 

 of these muscles lie wholly within the tongue (intrinsic), and for 

 the most part, by their contraction, give rise only to alterations 

 in shape. The extrinsic muscles have their point of attachment 

 outside the organ, and so are capable of causing alterations in 

 position as well as in form. 



Intrinsic Muscles. 



1. Superior longitudinal, pulls tip upwards and decreases length of 



dorsum. 



2. Inferior longitudinal, pulls tip downwards and inwards, i.e. curves 



dorsum. 



3. Vertical, working in conjunction with the transverse they produce 



a concave surface on the dorsum. Acting alone a convex surface 

 is produced. 



4. Transverse. 



Extrinsic Muscles. 



1 . Genio-glossus downwards. 



2. Hyo-glossus backwards. 



3. Chondro-glossus (not always present) backwards and downwards. 



4. Stylo-glossus backwards and towards palate. 



5. Palato-glossus side to side continuous with intrinsic transverse. 



