LUBRICATING TISSUES 389 



5/&, lubrication of parts of the body with an oil produced by a 

 gland : the sebaceous glands of the cat and the oil glands of the fowl. 



6th, the lubrication of the eye in the alligator and mouse. 



jth, the lubrication of the joints of a cat's bones by the synovial fluid. 



8th, the sweat glands and wax glands of the mammals. 



The Mucous Tissues of the Clam, Mya arenaria. If the mantle-fold 

 of a living example of this mollusk be drawn apart and the surface of 

 the inner sides of the mantle as well as the surface of the foot be exam- 

 ined, it will be noticed that bits of paper, particles of dust, and any other 

 small objects that may be dropped on this surface will move along, 

 always in a definite direction or path. A mapping out of all the paths 

 so determined will show that they form larger courses, of various curves 

 and straight runs, all tending toward and uniting to end at the labial 

 palps, and then passing from these to the mouth. 



A section of one of these regions (Fig. 353) will show two facts; the 

 presence of many mucous cells among a vastly greater field of ciliated 

 cells which, in life, are always 

 moving their cilia at any one 

 point in the same direction, and 

 that direction the same as one of 

 the paths. Thus, it is the cilia 

 that furnish the motive power, 

 but the food and the dirt par- 

 ticles WOUld not Cling tO the cilia FIG 353. Body epithelium of the clam, Mya. 

 The majonty of the cells bear the numerous 



alone especially Under Water. cilia on their distal surface, b.m., basement 

 Here the mUCOUS Cells Of the in- membrane; mu.c., mucous cells with round 

 . . . bodies and flat, crescentic nuclei. 



tegument play their part by pro- 

 viding a thin viscid covering to the ciliated surface. To this sticky 

 surface the particles of food as well as the particles of sand, dirt, 

 etc., in the water stick and the whole mass is thus carried along and 

 finally engulfed in the mouth. The lubrication cells (which here pro- 

 duce mucus) do comparatively little of what we might strictly call 

 lubrication. This is especially true of some of the fixed plecopods, as 

 the oyster, in which there is no active foot and the animal scarcely 

 moves in the quiet recess of its shell. In other forms that lead an 

 active life, especially the kinds that burrow rapidly in the sand, as 

 Unio, Mactra, or Ensatella, there is a considerable amount of contact 

 and abrasion between the strong, muscular foot, the 'mantle and other 

 parts, and here the mucus acts as a viscid, sliding buffer between the 

 delicate surfaces. In other words it acts as a true lubricant. Mya is 

 a form that, so far as motion and a consequent lubrication is concerned, 

 occupies a position about midway between the two. 



The mucous cells lie singly in different parts of the epithelium and 



