ANATOMY OF THE NERVE CENTRES. 451 
gly to gy. 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}y, and in small finches 3!5 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 gs'9y tO 150 and in 
Vertebrates, according to Cuvier, from ;'5 in some small birds 
to srkoo in 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-cesophageal centre 
is readily distinguished, giving origin to the maxillary nerves 
