GENERAL PHYSIOLOGY OF THE EPITHELIUMS. 195 



(between the buccal mucus and the saliva, for instance). 

 The serosities, found more or less abundantly in the serous 

 cavities, are produced in this way ; the synovia is the result 

 of fusion of the epithelial articulating membrane ; in order 

 to multiply the surfaces on which this fusion takes place, the 

 epithelium of the serous membranes has a great tendency to 

 vegetate, and thus are formed the epiploic appendages of the 

 peritoneum and the synomal fringes of the articulating cavi- 

 ties. The fluid produced by these surfaces serves to lubri- 

 cate them ; and, as it is observed that the serous cavities show 

 a tendency to disappear when they cease to be the seat of 

 motion, we may infer that their presence is a certain sign of 

 the existence of movements between the surfaces which they 

 line: therefore there must be movements, though they are 

 hardly perceptible, between the dura-mater and the arach- 

 noid, these two membranes being lined by a similar epithe- 

 lium to that of the serous membranes. 



All the waste of the epitheliums cannot, like the epidermal 

 scurf or the mucus, be carried to the exterior, or into the 

 cavities, like the synovia which is, however, partly reab- 

 sorbed. Besides, to carry off the waste part of the cells which 

 are placed in the deeper layers, a special apparatus is needed ; 

 this is supplied by the origin of the lymphatic system. The 

 lymphatic apparatus is composed of a -system of vessels, 

 which, if brought together in a diagram similar to that of the 

 blood-vessels, exhibits the form of a cone the summit of 

 which joins the venous system (thoracic duct and great 

 lymphatic vein connecting with the subclavians), while the 

 base (capillaries) is in contact with the epithelium (Fig. 

 60). The origin of the lymphatic capillaries is still little 

 known ; but it is probable that their primitive network is so 

 superficial that the base of the lymphatic cone may be con- 

 sidered as closed by the epithelial membranes ; * thus, when 

 any substance is placed in the skin, it is, as it were, placed 

 in the origin of the lymphatic system, whence its rapid 

 absorption ; in short, it is inoculated, and, mixing with the 

 lymph, flows with it into the circulating current. The lymph, 



1 Lately, however, lymphatic spaces have been discovered 

 situated around the smaller capillaries, and hence have been called 

 perivascular spaces. These may be considered as the origin of the 

 lymphatic system. These perivascular spaces, at first having no 

 limiting membrane, gradually coalesce, and form a small lymphatic 

 vessel having a true limiting membrane. [Am. ed.] 



