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 

 visdom of God that can be found in creation. It must 

 be observed, however, that the various portions are seen 

 in various degrees of development in different 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 vertebrse, 

 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. 



