ANATOMY OF THE NERVE CENTRES. 45 ' 



gV to -jV- These numbers show that the proportionate weight 

 of the brain is greater in small than in large animals. Thus, 

 in the goose the brain is 3^, and in small finches ;; V or more 

 of the body-weight. In the same class and amongst animals 

 of nearly the same bulk the proportionate size of the brain 

 varies perhaps with their intelligence and skill ; but even this 

 cannot be certainly affirmed, except approximately. The 

 statements are, however, worthy of being remembered, as 

 great misconception exists as to the proportionate develop- 

 ment of the nervous system in Insects as compared with that 

 of Vertebrates. In Insects the weight of the brain as com- 

 pared with that of the body varies from o-^Vo to T i-o, and in 

 Vertebrates, according to Cuvier, from T V in some small birds 

 t Trrihro- m some of the larger fishes, as in the Tunny 

 (Thynnus). Of course, all such estimates are approximate, 

 as the nervous system is always proportionately larger in 

 young animals ; and it is not possible to make due allowance 

 for bone, fat, chitin, and other tissues which are either feebly 

 supplied by nerves, or are extra nervous. 



The External Form of the brain of the Blow-fly imago is ex- 

 ceedingly simple, as all the principal fissures and divisions 

 between the lobes and lobules of the central medullary mass 

 are filled with cortical substance. The cephalic nerve centres 

 are somewhat flattened from before backwards, and consist of 

 a central portion, formed of two lateral halves, which is sub- 

 spheroidal, with the two mammiform antennal lobes in front, 

 and the great wing-like optic ganglia on either side, extending 

 to the inner surface of the dioptra of the great eyes. 



The form of the central medulla is, however, sufficiently 

 complex, as will be seen by a reference to the plates which 

 represent the parts in section, and to the stereographic figures 

 in Plate XXVIII., which have been constructed from a careful 

 study of numerous sections. 



The brain is perforated by the oesophagus, and it will be con- 

 venient to describe the several parts in reference to this open- 

 ing. Below the cesophageal canal the infra-ossophageal centre 

 is readily distinguished, giving origin to the maxillary nerves 



