MOTOR TISSUES. 33 



tion according to the law of gravitation. In the liv- 

 ing Body, as elsewhere in the universe, we can study 

 phenomena and make out their relations of sequence or 

 co-existence;, but why one phenomenon is accompanied by 

 another, why in fact any cause produces an effect, is a 

 matter quite beyond our reach in every case; whether it be 

 a sensation accompanying a molecular change in a nerve- 

 cell, or the fall of a stone to the ground in obedience to the 1 

 force of gravity. 



7. Motor Tissues. These have the contractility of the 

 original protoplasmic masses highly developed. The more 

 important are ciliated cells and muscular tissue. The for- 

 mer 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. 47*) lining the in- 

 side 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. Mus- 

 cular 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 fibres which present cross-stripes when 

 viewed under the microscope (Fig. 53f), and is hence 

 known as striped or striated muscular tissue. The other 

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

 alimentary canal and some other hollow organs, and con- 

 sists of elongated cells (Fig. 55 J) which present no cross 

 striation. It is known as plain or unstriated muscular 

 tissue. 



The cells enumerated under the heading of "undiffcr- 

 entiated tissues" might also be included among the motor 

 tissues, since they are capable of changing their form. 



8. The Conductive Tissues. These are represented by 

 the nerve fibres, slender threads formed by modification 

 and fusion of cells, and having the conductivity of the 

 amoeboid cells of the morula highly developed; that is to 

 say, they readily transmit molecular disturbances. When 

 its equilibrium is upset at one end, a nerve-fibre will 

 transmit to its other a molecular movement known as a 



*P. 115. f P- 123 t P. 124:. 



