INTRODUCTION. 15 
The functions of the animal body are divided into two classes: 
The animal functions, or those proper to animals, that is to say, sensi- 
bility and voluntary motion. 
The vital, vegetative functions, or those common to animals and vege- 
tables, 7. e., nutrition and generation. 
Sensibility resides in the nervous system. 
The most general external sense is that of touch; it is seated in the 
skin, a membrane that envelopes the whole body, which is traversed in 
every direction by nerves whose extreme filaments expand on the surface 
into papillz, and are protected by the epidermis and other insensible tegu- 
ments, such as hairs, scales, &c. &c. Taste and smell are merely delicate 
states of the sense of touch, for which the skin of the mouth and nostrils 
is particularly organized: the first, by means of papillae more convex and 
spongy; the second, by its extreme delicacy and the multiplication of its 
ever humid surface. We have already spoken of the ear and the eye. 
The organ of generation is endowed with a sixth sense, seated in its in- 
ternal skin; that of the stomach and intestines declares the state of those 
viscera by peculiar sensations. In fine, sensations more or less painful 
may originate in every part of the body through accident or disease. 
Many animals have neither ears nor nostrils, several are without eyes, 
and some are reduced to the single sense of touch, which is never absent. 
The action received by the external organs is continued by the nerves 
to the central masses of the nervous system, which, in the higher animals, 
consists of the brain and spinal marrow. The more elevated the nature of 
the animal, the more voluminous is the brain and the more is the sensitive 
power concentrated there; the lower the animal, the more the medullary 
masses are dispersed, and in the most imperfect genera, the entire nervous 
substance seems to melt into the general matter of the body. 
That part of the body which contains the brain and principal organs of 
sense, is called the head. 
When the animal has received a sensation, and this has occasioned vo- 
lition, it is by the nerves, also, that this volition is transmitted to the 
muscles. 
The muscles are bundles of fleshy fibres whose contractions produce all 
the movements of the animal body. The extension of the limbs and every 
elongation, as well as every flexion and abbreviation of parts, are the ef- 
fects of muscular contraction. The muscles of every animal are arranged, 
both as respects number and direction, according to the movements it has 
to make; and when these motions require force, the muscles are inserted 
into hard parts, articulated one over another, and may be considered as so 
many levers. These parts are called bones in the vertebrated animals, 
where they are internal, and are formed of a gelatinous mass, penetrated 
