DISTINCTIVE CHARACTERS OF ANIMALS. 29 
the central masses of the nervous system. Thus/we desire 
to perform a certain movement or set of movements; this 
desire leads to an act of volition or will; and the will causes 
a certain force or motor impulse to issue from the brain and 
travel along the nerves, so as to produce the desired motion, 
by exciting contractions in the muscles that perform it. Or, 
again, a certain sensation calls forth an emotion, which 
prompts a certain muscular movement, and may even cause it 
to take place against the will,—as when a strong sense of the 
ludicrous produces laughter, in spite of our desire (owing 
to the unfitness of the time and place) to restrain it; for 
the emotion, like the act of volition, produces a change in 
the nervous centres, which causes a motor impulse to travel 
along the nerves, and thus calls the muscles into contraction. 
And it seems to be in the same manner that those instinctive 
actions are produced, which, although few in adult Man when 
compared with those resulting from his will, predominate in 
his infant state, and through the whole life of the lower 
animals (Chap. xiv.). We shall also find that the nervous and 
muscular systems of animals are concerned in a class of actions 
with which the mind has no necessary connexion; these autom¬ 
atic actions, such as those of swallowing (§195) and breathing 
(§ 340), having for their object to assist in the performance of 
the organic functions, and to protect the body from danger. 
11. In the higher Animals, then, the presence of this 
Nervo-Muscular apparatus is an essential and obvious dis¬ 
tinction between their structure and that of Plants; and we 
find that it constitutes a large part of the bulk of the body. 
Thus the whole interior of the skull of Man is occupied by his 
brain; his limbs are composed of the muscles, and of the bones 
which support them and which are put in motion by them; 
and it is only in the interior of his trunk, that we find organs 
corresponding with those which form the entire fabric of the 
Plant. These organs of Nutrition have for their main pur¬ 
pose, to supply the wants of the organs of animal life; every 
exercise of which is accompanied by a certain decay or wear 
of their structure, and which consequently require to be con¬ 
tinually nourished and repaired, by the materials provided 
by what may be termed the vegetative organs. But in 
the lower of tribes of Animals, we do not find the animal 
functions to possess this predominance. In fact, among the 
