An Introduction to Zoology. 41 



In the lowest zoophytes, as we have no distinct vessels or muscles, we 

 have no distinct system of nerves ; here every thing partakes of a diffused 

 system, without definite organs ; we have diffused circulation, diffused con- 

 tractility, and diffused nervous influence, if we may apply that term when 

 distinct nerves there are none, and the only sense seems to be a diffused 

 sense of touch. But we have only to advance to the tribes of infusorial 

 animals, animals, with a few exceptions, invisible by the naked eye, in 

 order to find a distinct though simple nervous system, composed of a cer- 

 tain number of filaments, supplying one or two of the principal organs, and 

 concentrated in a common ganglion. Advancing further, to the very lowest 

 orders of mollusca, we trace very distinct filaments, connected with longer 

 knots or ganglions of nerves, each ganglion probably forming a distinct 

 centre of nervous influence and sensation, and in truth a little brain. 



In the bivalve mollusca, which have a distinct head, and organs at least 

 of the sense of vision, there is a more developed cephalic ganglion, which 

 may be regarded as a specific brain j and this is considerably enlarged in 

 the cephalopoda, where we first observe organs for the additional sense of 

 hearing. 



In all the vertebrated animals, a long spinal nervous chord is safely con- 

 veyed in a protected channel hollowed in the back of the vertebrae. This 

 consists of two sets of nervous filaments, the posterior sending off nerves, 

 which are found to be essential to the sensibility of the parts they supply, 

 while the nerves arising from the anterior filaments of the spinal chord, 

 are found to be those on which the power of motion depends in the volun- 

 tary muscles. The cephalic ganglion here also becomes greatly enlarged, 

 and formed into a regular brain, which in all the vertebrata presents the 

 same component parts, though developed in very different proportions in 

 the different orders. In all, an expansion of the anterior extremity of the 

 spinal chord, (the medulla oblongata,) supports a central tubercle, the 

 cerebellum, on which rest the two optic tubercles, above these the two 

 cerebral hemispheres are developed, in front of which are the olfactory 

 tubercles. 



It is the increasing development of the cerebral hemispheres, which 

 marks the progressive advancement of the orders. In fishes, these arc 

 among the most insignificant of the parts ; in reptiles they become far 

 more prominent j in birds they are again enlarged j and in the mammalia, 

 and especially in man, they attain their maximum of size. 



We have thus hastily traced the various functions necessary to maintain 

 each individual endowed with vital powers in a state of being and well- 

 being ; and we have endeavoured to lay down a concise, but we trust 

 comprehensive view of all the successive modifications of those functions, 

 throughout the various orders of the animal kingdom. It remains to con- 

 sider the general plan adopted to enable those individuals, destined them- 

 selves to a brief term of existence, yet to fulfil the first law of creative 



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