ALIMENTARY CANAL AND ITS MOVEMENTS. 257 



glandulse, yet there is strong reason to believe, from their position and 

 character, that it assists the pancreatic and biliary secretions in pre- 

 paring the alimentary materials for absorption (480). The glands of 

 Peyer are either solitary or ag ruinated ; the latter 

 form large patches, which are made of aggregations Fig. 70. 



of the former. Each solitary gland in its closed /j!**^ 



state consists of a spheroidal vesicle (Fig. 76, A), A ff r -'"'iV'^ 

 which is half imbedded in the mucous membrane, ^'J 



but which also forms an elevated projection above 

 it ; and this projection is surrounded by a ring or 

 zone of openings which lead into an annular cluster 

 of Lieberkiihnian follicles. On rupturing one of 

 these vesicles, its cavity is found to contain a gray- 

 ish white matter, interspersed with cells in various 

 stages of development ; and these products appear to 

 be set free by the spontaneous opening of the vesicle, as seen 5n section, after ha- 



, . , , , J , r , . , , r , Ting opened, with the folli- 



wnicn takes place when it has become mature, by cies of Lieberkuhn on either 

 the thinning away of its wall at its most projecting 

 part (Fig. 76, B). In any one of the agminated glands, some of the 

 vesicles are usually found to be open, and others closed. The closed 

 condition is not, as was once supposed, peculiar to the Peyerian glan- 

 dulae ; since, as will be shown hereafter, it is the general rule for 

 other glandular follicles in an early stage of their development to be 

 equally closed ( 718). Of the nature of the secretions of the Pey- 

 erian glandulse, nothing has been positively ascertained ; but some 

 probable inferences from well-known facts will be stated hereafter 

 ( T49). 



3. Movements of the Alimentary Canal. 



451. The food which is conveyed to the mouth, is grasped with the 

 lips, by a muscular effort, which is voluntary in the adult under ordinary 

 circumstances, but which may be performed automatically when the 

 influence of the will is withdrawn ; in the infant, as among the lower 

 animals, the action seems purely automatic, the nipple of the mother 

 being firmly grasped by the lips when introduced between them, even 

 after the brain has been removed. By the act of mastication, which 

 then succeeds, the food is triturated and mingled with the salivary 

 secretion ; and is thus prepared for the further process of solution, to 

 which it is to be subjected in the stomach. The degree of this prepa- 

 ration, and the form of the instruments by which it is effected, vary in 

 different animals, according to the nature of the food. In those Garni- 

 vora whose aliment consists exclusively of flesh, very little mastication 

 is necessary, because this substance is very readily acted on by the 

 gastric fluid ; and we accordingly find the molar teeth raised into sharp 

 cutting edges, and working against each other with a scissors-like 

 action (the only one permitted by the articulation of the jaw), so as 

 simply to divide the food. On the other hand, in those Herbivora 

 whose food consists of tough vegetable substances, such as the leaves of 

 grasses, or the stems and roots of other plants, we find the molar or 



17 



