388 HISTOLOGY 



and the making of dwelling cavities for the entire animal. The specific 

 cells of this tissue are not to be confounded with those of the serous tis- 

 sues which produce their secretion in a similar way. This method of 

 lubrication is mostly characteristic of animals that live in the water 

 although many of these spend large periods of their lives in the air. 

 Here, however, they must be in a damp location, for it is true of mucus 

 that it will not withstand a thorough drying. 



2. A tissue that secretes an oily substance called the sebaceous fluid, 

 whose primary use is to lubricate the skin and its appendages, and to 

 preserve them from the effects of drying. Among its principal secondary 

 uses is the production of odors. 



This tissue is found in those animals that live in the air, although 

 it has persisted in some of those that have later adapted themselves 

 to the water. The only alternative open to an animal that lives 

 exposed to the drying effects of the air and sun is to have a hard, 

 impervious covering of cuticle as in some of the Insecta, Crustacea, 

 and lower vertebrates. These animals dry very rapidly when confined 

 without a supply of water, showing that their cuticle does not entirely 

 protect them. 



3. A tissue that furnishes a "serous" secretion for lubrication pur- 

 poses. This form is somewhat rare, and is usually found in parts where 

 the surface to be lubricated is internal, as in the joints between the bones, 

 or semi-internal, as on the surface of the eye. In this last case the effects 

 of drying are avoided by the frequent renewal of the serous fluid by the 

 act of "winking" or rapid closing of the eyelid, thus carrying the fluids 

 over the surface of the eyeball. 



These fluids are of several kinds, and originate in several different 

 secretory surfaces or in glands that communicate with these surfaces. 

 Some lubricating glands are distributed over certain surfaces of the 

 mammalian body and are called the sweat glands. Others are confined 

 to particular regions and perform very special duties as the wax glands 

 of the ear tube. To cover the field thus laid out we shall examine the 

 following cases. 



i5/, a general lubrication of the body of a water animal with mucus, 

 accompanied with the removal of dirt and collection of food: the clam, 

 Mya arenaria. 



2d, the production of mucin by a land snail, Mesodon, from cells of 

 extreme specialization. 



36?, a general and complete lubrication of the body, and the animal's 

 habitat with mucus : the earthworm, Lumbricus. 



4th, a general lubrication of the body-surface of an animal living in 

 the air, with mucus (mixed with a poisonous or offensive substance): 

 the toad, Bufo Americanus. 



