FISHES. 
255 
■work which carries the gills. 2. The ribs, which are in 
general developed in a ratio inverse to that of the hyoid 
arches. Secondly, we find two pairs of limbs, each con- 
sisting of several pieces articulated to each other, and free 
at one extremity, while the other is jointed to the spinal 
column, or suspended in the muscles of the body. 
Such is a brief enumeration of the essential parts of an 
internal skeleton, which, when examined in detail, with 
intelligence of the purpose which every part is intended 
to subserve, forms one of the noblest monuments of the 
wisdom of God that can he found in creation. It must 
be observed, however, that the various portions are seen 
in various degrees of development in diflerent classes of 
animals, and that some of the constituents are occasionally 
either very rudimentary or entirely wanting. 
Not less important in the economy of a vertebrate 
animal is the condition of the nervous system. To this, 
indeed, the skeleton is ancillary. There is a great concen- 
tration of nervous matter in the fore part of the animal, 
constituting the brain, whence cords are given forth to the 
organs of sense, which are mostly situated in the vicinity. 
From its hinder part proceeds the spinal marrow, running 
along a tube formed by the superior arches of the vertebra;, 
and giving off a number of threads on each side in sym- 
metrical pairs. The spinal marrow itself is not homoge- 
neous, but is composed of four cords fused together, of 
which the upper pair is destined to convey the commands 
of the will to the voluntary muscles, while the lower pair 
receives the sensations which are conveyed from without. 
Hence they are respectively termed the motor and the 
sensitive tracts of the spinal cord. 
