16 ANIMAL PHYSIOLOGY. 



This is what is called reflex action (p. 17 8), and in such a 

 case the part irritated, from which the nervous impulse 

 starts, is still said to have sensibility, and the nerve to be 

 sensory, although there is no sensation, and the movement is 

 involuntary. Also, the property of response to irritation is 

 not confined to the nervous system ; structures may alter 

 their shape or undergo other change on application of a 

 stimulus, and this property is termed irritability. The 

 active part of change of shape or movement probably in all 

 cases consists in contraction, and is hence called contrac- 

 tility. 



Irritability and contractility, although they may well be 

 included under the terms sensory function and movement, 

 are not, like sensation and voluntary movement, confined to 

 animals. They are found in the vegetable world also; and 

 it may be maintained with probability, that they are proper- 

 ties of every living part of every living being. 



6. The expression living parts of living beings, has been 

 already twice used, and will attract the student's attention 

 to the fact that every part of the texture of the body does 

 not equally exhibit the phenomena of life. In a large 

 majority of the different textures, a considerable or even the 

 greater part of the bulk is composed of mere deposited 

 matter, which, although it undergoes both structural and 

 chemical changes, offers no sufficient evidence of the posses- 

 sion of properties peculiar to living beings; but, imbedded in 

 this, or in other instances forming the principal mass of the 

 texture, there is always to be found a set of elements which 

 exhibit some or all of the four functions nutrition, repro- 

 duction, contractility, and irritability. 



These living elements of texture always consist of material 

 belonging to one chemical group of substances ; namely, those 

 which are termed sometimes the proteids, but which may 

 probably be more conveniently distinguished as the albumi- 

 noids, albumen and fibrin being among the most familiar 

 examples of them. The substances of this group are the 

 most complex combinations of carbon, hydrogen, oxygen, 

 and nitrogen; and, as they are found in nature, contain 

 also phosphorus, sulphur, potash, and soda. 



Yery frequently the expression protoplasm is used to in- 



