DIGESTION. 241 



concerned in the production of the second act, the actual discharge of 

 the materials of secretion, together with a considerable amount of fluid, 

 the latter being an actual secretion by the protoplasm, as it ceases to oc- 

 cur when atropine has been subcutaneously injected. 



In the mucous-secreting gland, the changes in the cells during secre- 

 tion have been already spoken of (p. 233). They consist in the gradual 

 secretion by the protoplasm of the cell of a substance called mucigen, 

 which is converted into mucin. and discharged on secretion into the canal 

 of the alveoli. The mucigen is, for the most part, collected into the 

 inner part of the cells during rest, pressing the nucleus and the small 

 portion of the protoplasm which remains, against the limiting membrane 

 of the alveoli. 



The process of secretion in the salivary glands is identical with that 

 of glands in general; the cells which line the ultimate branches of the 

 ducts being the agents by which the special constituents of the saliva are 

 formed. The materials which they have incorporated with themselves 

 are almost at once given up again, in the form of a fluid (secretion), 

 which escapes from the ducts of the gland; and the cells, themselves, 

 undergo disintegration again to be renewed, in the intervals of the ac- 

 tive exercise of their functions. The source whence the cells obtain the 

 materials of their secretion, is the blood, or, to speak more accurately, 

 the plasma, which is filtered off from the circulating blood into the in- 

 terstices of the glands as of all living textures. 



THE PHARYNX. 



That portion of the alimentary canal which intervenes between the 

 mouth and the oesophagus is termed the Pharynx (Fig. 164). It will 

 suffice here to mention that it is constructed of a 

 series of three muscles with striated fibres (con- 

 strictors), which are covered by a thin fascia ex- 

 ternally, and are lined internally by a strong fas- 

 cia (pharyngeal aponeurosis), on the inner aspect 

 of which is areolar (submucous) tissue and mu- 

 cous membrane, continuous with that of the 

 mouth, and, as regards the part concerned in swal- 

 lowing, is identical with it in general structure. Fia . ^Lingual foi- 

 The epithelium of this part of the pharynx, like \\ cle or crypt, a, invoiu- 



* a * tion of mucous mem- 



that of the mouth, is stratified and squamous. rane with its papillae; 



6, lymphoid tissues, with 

 The pharynx IS Well Supplied With muCOUS several lymphoid sacs. 



glands (Fig. 182). 



The Tonsils. 



Between the anterior and posterior arches of the soft palate are situ- 

 ated the Tonsils, one on each side. A tonsil consists of an elevation of 

 16 



