24 THE FUNCTIONS OF ANIMALS. 
others depend for their carbon supplies on the sugar, starch, 
and fat already made by other animals, or by plants. As 
regards nitrogen, most plants take this from nitrates and 
the like, absorbed along with water by the roots; whereas 
animals obtain their nitrogenous supplies from the complex 
proteids formed within other organisms. Most plants, 
therefore, feed at a lower chemical level than do animals, 
and it is characteristic of them that, in the reduction of 
carbon dioxide, and in the manufacture of starch and 
proteids, the kinetic energy of sunlight is transformed by 
the living matter into the pctential chemical energy of 
complex foodstuffs. Animals, on the other hand, get their 
food ready made; they take the pounds which plants have, 
as it were, accumulated in pence, and they spend them. 
For it is characteristic of animals that they convert the 
potential chemical energy of foodstuffs into the kinetic 
energy of locomotion and other activities. In short, the 
great distinction—an average one at best—is that most 
animals are more active than most plants, 
Chief functions of the animal body.—We have seen that 
there are two master activities in animals, those of muscular 
and of nervous structures, and that the other vital processes, 
always excepting growth and reproduction, are subservient 
to these. Let us now consider the various functions, as 
they occur in some higher organism, such as man. 
Nervous activities—Life has been described as consisting 
of action and reaction between the organism and its en- 
vironment, and it is evident that an animal must in some 
way become aware of surrounding influences. 
The unit in nervous reaction in any highly organised 
animal is the vefex. It requires three structures, a receptor 
(end-organ), a conductor, and an effector (muscle). The 
conductor consists of two or more nerve cells or neurones 
which span the distance between receptor and effector by 
means of their long processes. Stimulation of the receptor 
causes a nervous impulse to be transmitted along the 
conductor to set the effector in action, The whole nervous 
system is essentially a connected series of such reflex-arcs, 
all intricately joined up with one another. 
There are two chief kinds of stimuli which are transmitted 
to the central nervous system—stimuli from without the 
