CHANGES OF THE FOOD. 7 



This continually separates, particularly under friction, in the 

 form of laminated particles like scales, more or less mingled 

 in general with fluid matters. Yet when rubbing, without 

 moisture, is employed, the particles fall off in a dry state. This 

 epithelium is remarkable as having neither nerves nor blood- 

 vessels, and as being wholly destitute of sensibility. It is a 

 secretion from the subjacent blood-vessels or, to speak more 

 correctly, it continually renews itself by drawing material from 

 the subjacent blood-vessels, as the outer layers of cells are 

 thrown oif. The same kind of structure, under the name epi- 

 thelium alone that is, not epidermis also covers the free sur- 

 faces of the internal cavities in general ; while the outer layer 

 of cells is in like manner continually thrown off in the form of 

 flattened cells like scales, and becomes mingled with the several 

 secretions proper to these internal cavities. For example, an 

 epithelium, variously modified, exists as on the skin, so on the 

 mucous membranes ; on the serous membranes, such as those 

 which line the shut cavities of the chest and abdomen ; on the 

 synovial membranes, which line the cavities of the joints ; and 

 on the lining membrane of the blood-vessels and lymphatics. 

 It is common to the epithelium of all these parts, as well as to 

 that of the outer surface, that it consists of cells united together 

 by a more or less" cohesive matter ; that the cells make their 

 first appearance at the deepest part of the structure, and after 

 undergoing various changes of form, are finally thrown off, to 

 be succeeded by others from below. 



To obtain a just idea of the nature of the changes which the 

 food has to undergo before it is rendered fit to become incor- 

 porated with the body, so as to augment and repair its fabric, 

 and perform whatever other uses it may be discovered to pos- 

 sess, it has to be traced through a long series of organs. Of 

 these the mouth stands first. In the mouth the food is mas- 

 ticated by the teeth, and mingled with the saliva derived from 



