200 COPE. [Vot. III. 
V. Proximal phalangeal keels, from impacts derived from 
apex of unguis, torsion, and pressure of single median tendon 
and sesamoid bone on distal end of proximal phalanges. 
VI. Astragalo-tibial keel ; impact and torsion aided by gravity. 
VII. 7Ytbi0-astralagar keels ; from pressure of dense edges of 
astragalus on spongy end of tibia, with impacts and torsion. 
Mi THE AXIS OF THE SEELETOMN: 
The discussion of this part of the skeleton falls naturally into 
the departments of the skull and of the vertebral column. 
I, DHE SKUEE: 
a. The Sense Organs. 
The modifications in the form of the skull are generally di- 
rectly related to the muscular strains to which it is subjected. 
The size of the brain cavity is directly as the development of 
the brain, especially of the hemispheres. The size of its sense 
capsules is directly as the activity of the sense which they sub- 
serve. With diminution of the sense of sight the definition 
of the osseous orbits grows less and disappears, as in the mole 
and other subterranean species. Nocturnal species which are 
constantly stimulated by diminished, but not extinguished light, 
have acquired large eyes, which occupy large orbits. Species 
in which the sense of smell is atrophied have very small nasal 
chambers, as the Cetacea. 
As regards the modification in the proportions of the skull 
in adaptation to the brain and organs of special sense, they can- 
not be said to be due to direct mechanical interference of the 
latter, in impressing the former. The changes in proportions 
are here effected during embryonic life by the change in the 
distribution of building material. The brain and organs of 
sense are outlined in the embryo before the skull has attained 
any rigidity, and the latter may be truly said to be moulded on 
the former. We have here an illustration of the effect of 
use which is not mechanical, since the softer tissue of the 
brain and sense organs cannot coerce the harder tissue of 
the skeleton. It shows how that use, whether it effect mechan- 
ical changes in the hard tissues or not, produces a distinct 
