INDIVIDUAL VARIATION IN EQUUS ASINUS. 719 
assists in the support of os innominatum; and in other respects 
the “ true sacral bones ” are not always the same. 
Coccygeal. —Vary in number, especially with age. 
The peculiar modifications which we have noted in the seventh 
cervical, the uncertainty observable in the anterior and posterior 
parts of the lumbar and sacral regions, and the variation in the 
number of ribs prominently bring to our mind the question “ How 
is the vertebral column becoming modified in the present day?” 
and also, “Will an examination of these points throw any light 
on the remarkable preponderance of dorso-lumbar vertebrae and 
costae in Perissodactyla ungulata ?” 
Skull. —-Exhibits many minor variations, most of them pro¬ 
bably sexual or due to age. 
Limbs , fore.—Scapula: differences in figure, thickness, &c. 
Often, instead of the gradual disappearance of the spine interiorly, 
it terminates in a slightly prolonged -process, a rudimentary 
acromion similar to that seen in the ox, but smaller, Medullary 
foramen varies in position; also glenoid cavity rounded or oval. 
Humerus: synovial fossettes vary in size and form, as do 
those in upper part of radius and ulna. The ulna generally 
extends downwards only two thirds of the length of the radius; 
but in the foetus it is much longer in proportion, and in the 
adult we occasionally find it passing downwards to the supero- 
external part of the knee to articulate with os cuneiforme. If 
we examine the inferior extremity of the radius of the foetal colt, 
we may note that at its external part is a small ossific centre. 
Our cases of variation show us that we must describe this as 
the inferior extremity of the ulna. The value of this observation 
as illustrating the similarity of the forearm of the ass to that of 
the ox is evident. 
Carpus: os trapezium v. pisiforme generally present, but 
sometimes absent. 
Metacarpus and phalanges: major variations do not come 
under our notice (in the ass). 
Hind. —Os innominatum : form, size, and relations of auricular 
facet of ilium vary. 
Fibula varies similarly to ulna in fore limb. Often extends to 
tarsus, occupying whole length of outer part of tibia. In other 
cases, and generally, consists superiorly of a small button-shaped 
appendage externally placed to upper part of tibia, tapering to a 
point interiorly, from which a white fibrous band extends down¬ 
wards, becoming continuous with the inferior extremity of the 
tibia, the external part of which is developed from a distinct 
ossific centre, the representative of the tarsal fibula of ruminants. 
Tibia and astragalus: synovial fossettes vary in develop¬ 
ment. 
