SUMMARY. 243 



common sensitive or motive nerves, and it does not account sufficiently for their more 

 or less exalted sensitive or motive properties in different animals. The structure of the 

 common sentient organs and the muscles, the size of their nerves and the extent of 

 their local origins, give the peculiarity of each degree of sense and of the motive 

 power, but the continuations of the tracts from the local origins and the extent of their 

 communications with the convoluted surface of the brain, give the sensorial quality of 

 each sense, and the connexions of the motive tracts with the convolutions determine 

 the variations of voluntary motion. According to the extent of the local centres, and 

 their connexions with the convolutions, sensations vary in every degree, from a simple 

 perception to the highest quality ever experienced. Voluntary motion may also vary 

 in mere quickness or power up to the accomplishment of the most correct and graceful 

 evolutions. It must be apparent that, however great the resemblance of the form of 

 animals may be, and of the organs belonging to them, and even of the parts of the 

 nervous system supplying them, in such respects only they approach each other for the 

 performances of similar animal functions. The greater difference in the scale of 

 creation must be looked for in the larger proportions of the brain, by which all their 

 powers become gradually increased and brought nearer to the higher qualities of the 

 intellect. It is probable that for the higher sensitive or motive qualities of particular 

 organs required for the instinct in fulfilling the special purposes of some creatures, very 

 minute sensorial centres are present in the brain. 



Besides the previous acquaintance with the centres and nerves, it may be proper 

 to notice some points relating to their texture, and their inherent and acquired 

 properties. 



The investing membranes are generally similar in each class of animals, but there 

 are deviations which partake of the peculiarities of other parts of the body, as in the 

 colour of the skin and hair, and the pigment of the eye ; there are also modifications of 

 texture. Most commonly a nerve is white and glistening, but has sometimes a tint 

 inclining to yellow or blue. In the pelican, the nerves supplying the beak are covered 

 with a black membrane. It has different colours in invertebrate animals, and is not 

 always similar in the same species of the vertebrate, but varies like the black or dark 



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