TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 33 



are the 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 ex- 

 ample (Fig. 46), 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 three 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 micro- 

 scope (Fig. 42), 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. A 

 second kind of muscular tissue is found in the walls of the alimen- 

 tary canal, the arteries, and some other hollow organs, and consists 

 of elongated cells (Fig. 44) which present no cross-striation. It is 

 known as smooth or unstriated muscular tissue. The third sort 

 occurs only in the heart. It consists of branched cells presenting 

 some indications of cross-striation (Fig. 45) and is called cardiac 

 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 covering the whole exterior of 

 the Body and forming 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- 

 excretory or receptive; and cilated cells 



face of the peritoneum, have already been included among the motor 



a, cell-body ; c, nucleus. , . .-,-,, . . , . i_ T_ 



tissues. The protective tissues may be best 

 defined as including cells which cover free surfaces, and whose 

 functions are mainly mechanical or physical. In their simplest 

 form epithelial cells are flat scales, as, for example, those repre- 



