CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 515 



minute vortices. Blood injected into the peritoneal cavity of an 

 animal, disappears, is "absorbed," at least under favourable cir- 

 cumstances ; and, in some cases at least, the blood so injected 

 has been observed to pass away into the lymphatics, presumably 

 by the above mentioned stomata, and so to find its way into the 

 blood stream. 



By similar stomata the pleural cavity is put into communi- 

 cation 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 Peyer's 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 mucosse. 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." 



