34 



THE HUMAN BODY 



protoplasmic masses highly developed. The more important are 

 ciliated cells and muscular tissue. The former line certain surfaces 

 of the Body, and possess on their free surfaces fine threads which 

 are in constant movement. One finds such cells, for example 

 (Fig. 40), lining the inside of the windpipe, where their threads or 

 cilia serve, by their motion, to sweep any fluid formed there 

 towards the throat, where it can be coughed up and got rid of. 

 Muscular tissue occurs in two main varieties. One kind is found in 

 the muscles attached to the bones, and is that used in the ordinary 

 voluntary movements of the Body. It is composed of fibers which 

 present cross-stripes when viewed under the microscope (Fig. 45), 

 and is hence known as striped or striated muscular tissue. Because 

 the muscles which are made of this sort of tissue are attached to 

 bones they are often called skeletal muscles. The other kind of 

 muscular tissue is found in the walls of the alimentary canal, the 

 arteries, and some other hollow organs, and consists of elongated 

 cells (Fig. 47) which present no cross-striation. It is known as 

 smooth or unstriated muscular tissue. 



The cells enumerated under the heading of " undifferentiated 

 tissues" might also be included among the motor tissues, since 

 they are capable of changing their form. 



8. PROTECTIVE TISSUES. These consist of certain cells lining 

 cavities inside the body and called epithelial cells, and cells cover- 

 ing the whole exterior of the Body and form- 

 ing epidermis, hairs, and nails. The enamel 

 which covers the teeth belongs also to this 

 group. 



The class of protective tissues is, however, 

 even more artificial than that of the nutritive 

 tissues, and cannot be defined by positive 

 characters. Many epithelial cells are secre- 

 tory, excretory or receptive ; and ciliated cells 

 have already been included among the motor 

 tissues. The protective tissues may be best 



FIG. 12. Flat epi- 

 thelium cells from the 

 surface of the perito- 

 neum, a, cell-body; defined as including cells which cover free 



c, nucleus; b, nucleoli. . , 



surfaces, and whose junctions are mainly 

 mechanical or physical. In their simplest form epithelial cells are 

 flat scales, as, for example, those represented in Fig. 12, from the 

 lining membrane of the abdominal cavity. 



