INTRODUCTORY. 
15 
namely, I. Vertebrated animals; IT. Molluscous animals ; 
III. Articulated animals; and, IV. Radiated animals. 
The foundation of these divisions rests on the organization of 
the various animals, as they exist in nature. 
All animals are characterised by sensation and motion. The 
brain and nervous system are the medium by which the func¬ 
tions of animal life is manifested : while the heart and its acces¬ 
sory organs, nutrition and generation, &c. are the vital and vege¬ 
tative functions, and are common to animals and plants. Sen¬ 
sation, therefore, exists in the nervous system. As we descend 
in the scale of being, these agents gradually become less perfect, 
until they at last disappear. In the lowest state of animal ex¬ 
istence, the nervous system is invisible, if it exists at all; and 
the muscular fibre has given way to a shapeless mass of animal 
matter. 
OF VERTEBRATED ANIMALS. 
From man, who stands at the head of vertebrate animals, in 
consequence of his perfect organization, down to the lowest of 
the fish, the brain is encased in a cranium, and the spinal cord 
is inclosed in a bony articulated column, called the spine. To 
the sides of the spine the ribs are attached, and the bones of the 
upper and lower extremities: these are either articulated, or 
kept together by ligaments. Over these, again, are placed the 
muscles, which give action to the bones. The alimentary sys¬ 
tem is inclosed within the cavity of the ribs and abdomen. 
All vertebrated animals have a muscular heart, red blood ; 
the mouth is constructed with two horizontal jaws ; and organs 
are situated in the head by which they enjoy, through the me¬ 
dium of the nerves, the various senses, namely, of vision, hear¬ 
ing, smell, and taste. There is no instance of their having 
more than four limbs. 
Comparative anatomy enables us to trace a similarity to all 
the parts of man, through the whole vertebrated animals. 
The organs of sense, in all animals with a spine, consist in 
two eyes, two ears, two nostrils, the tongue, and all the muscles 
which cover the skeleton. The nervous system takes its rise in 
two masses situated in the cavity of the skull; the substance is 
called medullary, or marrow; and anatomists have remarked, 
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