t 



NEEVOUS SYSTEM AND GE^EBAX SENSATION. 



[91. Comparative anatomy, and the history of 

 animal evolution, have shed an important light upon 

 the relative importance of the different masses that 

 compose the brain ; a general survey, therefore, of the 

 morphology of this organ may illuminate the stu- 

 dent's path, and enable him to comprehend more 

 clearly its complicated structure. 



[92. We can easily trace a progressive develop- 

 ment of the structure of the brain, in the entire series 

 L - of the vertebrated animals. In FISHES its consti- 

 tuent parts appear in the form of globular masses, 

 which lie behind each other on the same plane. The 

 volume of the brain is small in proportion to the mass 

 of the body; thus it is 1-720 in Gadus lata, 1-1305 

 in Esox Indus, 1-1837 in Silurus glanis, and only 

 1-37440 in Scommher thynnus. Its relative propor- 

 tion to the spinal cord is seen in the annexed figure 

 of the cerebro-spinal system of the bleak, Cypri.nus 

 alburnus (fig. 21), where a, is the ganglia of the 

 hemispheres ; b, is the optic lobes ; c, the cerebellum ; 

 d, the medulla oblongata; e, the spinal cord. The 

 cord presents anterior and posterior columns, as in 

 man, and enlarges into the medulla oblongata, which 

 may be regarded as an integral part of the brain ; 

 from it arises most of the cerebral nerves ; the cere- 

 bellum (c) is single, and occupies the median line ; it 

 exhibits various phases of development in the dif- 

 ferent families. In front of the cerebellum 

 we find a pair of ganglia the optic lobes 

 (b) which in bony fishes give origin to the 

 optic nerves ; they are hollow, and exhibit 

 internally the rudiments of parts that are 

 more fully developed in the higher classes ; 

 transverse bands of neurine unite these gan- 

 glia together. Before the optic lobes a se- 

 cond pair of ganglia are placed the cere- 

 bral hemispheres (a) ; they are small, and 

 he apart, but are united by a transverse 

 band in bony fishes : with these masses the 

 olfactory nerves (fig. 22, 1) are connected, 

 which sometimes form ganglia before they 

 are distributed to the nose (figs. 22 and 23, 



Fig. 22. 



Fig. 23 



