CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 489 



By similar stomata the pleural cavity is put into communica- 

 tion with the lymphatics not only of the diaphragm (on its 

 pleural surface) but also of the lungs, and to a smaller extent of 

 the thoracic walls, and during the movements of the chest in 

 breathing the contents of the pleural cavity are continually being 

 pumped away, partly into the lymphatics of the lungs partly into 

 those of the diaphragm and chest walls. In a similar manner 

 pericardial fluid passes away from the pericardial cavity, and the 

 fluid in other smaller serous cavities such as that surrounding the 

 testis, passes away from the respective cavities into the general 

 lymphatics. The quantity of fluid in even the largest of these 

 cavities is at any one time in normal conditions very small, but 

 that fluid appears to be continually renewed, old fluid passing 

 away to the lymphatic system, and new fluid taking its place. 

 The serous cavities therefore are to be regarded as expanded 

 initial reservoirs from which as well as from the lymph-capillaries 

 and lymph-spaces of the tissues the lymph stream is continually 

 being fed. 



The Structure of Lymphatic Glands. 



290. Solitary Follicles and Peyers Patches. All along the 

 small intestine and at various points of the circumference are 

 found, partly in the submucous tissue but reaching up to the 

 surface of the mucous membrane, small rounded bodies, of the 

 size of a small pin's head, more numerous perhaps in the lower 

 than in the upper part of the bowel, often called 'solitary 

 glands.' They are not glands however in the sense ( 209) of 

 being involutions of the mucous membrane, and it is better 

 perhaps to speak of them as solitary follicles. At the free border 

 of the small intestine, opposite to the attachment of the mesentery, 

 the mucous membrane contains long oval patches, Peyer's patches, 

 placed lengthways, there being some twenty or thirty of these ; 

 they are most numerous in the ileum and disappear towards the 

 duodenum. Each patch is practically a group of solitary follicles, 

 and indeed these patches are sometimes spoken of as agminated 

 follicles. In the large intestine especially at the caecum, and in 

 man particularly in the vermiform appendix, solitary follicles are 

 abundant, but here they lie wholly in the submucous tissue below 

 the muscularis mucosae. In the stomach also, in young people, 

 there occur in the mucous membrane, generally between the 

 mouths of the glands, structures which are very similar to solitary 

 follicles and which are sometimes called " lenticular glands." 



A solitary follicle consists essentially of a spherical mass of 

 fine adenoid tissue the meshes of which are crowded with 

 leucocytes. In the intestine as we have seen ( 259) the con- 

 nective tissue lying between the epithelium above and the 

 muscularis mucosaa below has a reticular arrangement and 



