30 CLASSIFICATION 



Of the Third Class, or Fish. — This class of animals is a considerable 

 remove from the former in complication of structure. We have observed 

 that they have a complete circulation, making in the whole almost a 

 second. 



They are endowed with five senses. 



The brain in this class is, upon the whole, much larger in proportion 

 to the size of the animal than in the former. It is a very irregular 

 mass ; but the several parts that are similar to those in a still superior 

 order may be picked out. The brain varies in shape in this very much 

 more than in any other class of animals. The cerebrum in some, as 

 in the Skate, is detached to some distance from the other parts ; in 

 others it is pretty closely connected. There are more parts in some 

 than there are in others. 



They have a medulla spinalis, or continuation of the brain down the 

 back. 



In the first class we had the brain surrounded by soft parts only. In 

 the second it was closely surrounded by soft parts, but these were sur- 

 rounded by hard. In the present class the brain has a case of hard 

 parts for itself, called the skull ; but it is too large for the brain, there- 

 fore this is attached to the skull by a cellular membrane, which makes 

 a kind of tiinica arachnoides 1 . 



The nerves arising from the brain in this class are very large, and 

 there seem to be nine pairs. 



Of the Fourth Class, or Amphibia. — The brain in this class is very 

 small in proportion to the size of the animal, smaller than even what 

 it is in the former, or Fishes. 



It would seem from external appearance to be made up of many 



studied in relation to the modern discoveries of the functions of the several parts of 

 the spinal chord of the vertebrate animals. Mr. Newport (Phil. Trans. 1834, p. 40.3) 

 describes the abdominal cords of insects as composed of two tracts, a ganglionic or 

 sensitive which is anterior or ventral, and a motor tract which passes over the 

 ganglions on the posterior or dorsal aspect. In addition to these there is a narrower 

 column on the posterior part of the motor tract, which he calls the involuntary 

 tract, and would therefore more immediately answer to the sympathetic or great 

 intercostal nerve. These parts in the Crustaceans and in the imago of the Insect 

 are protected by a specially investing substance, of which Hunter appears to have 

 been aware from his assigning the floating or unprotected condition of the nervous 

 centres as a character of his first class.] 



1 [The mode of progression of fishes requires that the head should be of large 

 size, to divide the water and to afford adequate attachment to the mass of muscles 

 passing to it from the body. The mode in winch this is effected without incurring 

 an undue accumulation of ponderous matter about the brain, is now acknowledged 

 to be that which Hunter has described, viz. by an extraordinary development of the 

 arachnoid covering, the cells of which are filled with a serous fluid ; and upon this 

 i> the skull moulded.] 



