162 
Mr Manners-Smith , A Study of the Navicular 
by a groove into two facets, the groove was continuous with the 
suture on the astragalar surface. 
The large size of this process however in the cuboidal type 
of bone can be explained equally well mechanically as morpho- 
logically. It must be remembered in dealing with this bone, 
that it is later in its ossification than any of the other tarsal 
bones and that the child is walking, from, say the second to the 
fifth year, with its navicular in a cartilaginous condition. During 
this period, the inferior calcaneo navicular ligament is exerting 
traction on the plantar point, and in this way, supposing the bone 
ossifies late, and the child is particularly active, the point might 
easily be drawn out into a prominent process. Its nipple-like 
appearance in many cases certainly suggests this mode of origin. 
The plantar point is well developed in most cases ; very small 
in some of the specimens, and in a few absent. There are two 
types of plantar point, (a) the spur-like or nipple-like, ( b ) the 
rounded. It is spur-like or nipple-like in 318 specimens and 
rounded in 134. It appears to be better developed in these 
ancient bones than in modern specimens. 
The nature and depth of the concavity. This varies consider- 
ably in different specimens. In a few cases it is almost absent, 
the facies articularis posterior being nearly flat. There are two 
types of concavity. 
(a) That with an increase of the concavity in the long 
diameter, which depends upon the extent of projection backwards 
of the upper and outer part of margo dorsalis posterior (the 
margin of the broad end of the pear-shaped facet for caput tali), 
since the tibio plantar angle (narrow end of the pear) projects 
backwards considerably in nearly all cases. 
(i b ) That with an increase of the concavity in the shorter 
diameter, which in turn depends upon the projection towards the 
posterior surface of the plantar point. 
When the concavity was especially deep, the increase in most 
cases (238) was in the longer diameter only, in the shorter 
diameter only in 55, in both diameters in 82. It is interesting 
to compare the type of concavity in man with that existing in the 
Simiidae. In all four genera the depth of the concavity is chiefly 
in the long diameter and the plantar point does not extend 
towards the facies articularis posterior, but is small and is con- 
fined to the facies plantaris. Since movement at the astragalo 
navicular joint in the Simiidae is especially free, the view is 
rather suggested that a deeply concave surface for caput tali, in 
the long diameter in man, is also correlated with great freedom of 
movement in a lateral direction at the same joint, so that dis- 
location may be prevented during the side to side movement. 
The depth of concavity in the shorter diameter only, in man, 
