THE DIFFERENTIATION OF TISSUES. 35 



out. The highest manifestation of this latter faculty, shown 

 objectively by muscular movements, is subjectively known as 

 the " will," a state of consciousness; and other mental phe- 

 nomena, as sensations and emotions, are also associated with 

 the activity of nerve-cells lying in the brain. ^JHow it is that 

 any one state of a material cell should give rise to a particular 

 state of consciousness is a matter quite beyond our powers 

 of conception Abut not really more so than how it is that 

 every portion 01 matter attracts every other portion according 

 to the law of gravitation. In the living Body, as elsewhere 

 in the universe, we can study phenomena and make out their 

 relations of sequence or coexistence; but why one phenom- 

 enon is accompanied by another, why in fact any cause pro- 

 duces 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 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 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. 50), 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 mus- 

 cles attached to the bones, and is that used in the ordinary vol- 

 untary movements of the Body. It is composed of fibres which 

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

 56), and is hence known as striped or striated muscular tis- 

 sue. 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. 60) which present no cross-stria- 

 tion. It is known as plain or unstriated muscular tissue. 



The cells enumerated under the heading of "undiffer- 

 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, each of which has as its essen- 

 tial part a branch of a nerve-cell having the property of physio- 

 logical conductivity highly developed; the fibres therefore 



