(5) THE MASTER TISSUES —NERVE AND 



MUSCLE. 



By means of the epithelial and connective tissues the 

 body is protected, supported and nourished. It performs 

 purely vegetative functions, but it is not brought into active 

 relationship with its environments. By the development of 

 Nerve and Muscle the surroundings are able to act upon the 

 body, and the body can react upon its surroundings. 



These tissues may therefore be called the Master Tissues, 

 and it is as their servants that all the 

 other tissues of the soma functionate. 



Upon them the very existence of an 

 animal depends. It lives in a world of 

 constant change. The surrounding con- 

 ditions are not always compatible with life ; 

 the temperature may be too high or too 

 low, or it may find itself plunged in a 

 medium in which it cannot breathe and it 

 must escape ; food may be wanting, and it 

 has to be obtained. A thousand changfinsf 

 conditions have to be daily, hourly, almost 

 momentarily met by adjustments of the 

 body, and to make these appropriate the 

 various different kinds of change must 

 each produce different effects. 



Some means by which they can do so 

 is the first essential for the continuance of 

 the animals' existence. Not only must each produce its special 

 effect, but means must exist by which each different effect 

 or combination of effects may produce an appropriate 

 reaction. 



In unicellular organisms changes in the surroundings act 



51 



Fig. 16. — Poterio- 

 dendron in its cap- 

 sule, to illustrate 

 the first stage in 

 tlie evolution of 

 a neuro-niuscular 

 system. 



I 



