This disappearance of the lateral digits cannot excite surprise when we consider the functions of the organs. Becoming useless, they must undergo gradual atrophy from want of use.
There undoubtedly is, in this former existence of supplementary digits in the horse, something analogous to what we still find in the pig; where the two principal digits are accompanied by two shorter ones, which very probably, from their infrequent use, are destined to disappear in a more or less distant future.
Proportions of the Arm, the Forearm, and the Metacarpus
As a supplement to the study of the anterior limbs which we have just finished, it appears necessary to give some indications of the relative proportions of certain of the segments which form these limbs in the plantigrades, the digitigrades, and the ungulates.
First, we would remark that, in following this order of classification, the scapula becomes less and less narrow, and assumes a form more and more elongated. In order to convince ourselves of this, it will be sufficient to study the bone first in man, then in the bear, the cat, dog, ox, and finally in the horse.
As to the proportions of length, which are those we should chiefly study, we shall commence with the comparison of the forearm and arm--that is to say, the radius and the humerus. The radius is found to be longer in proportion to the humerus, as the number of digits is smaller, and the hand loses more and more the functions of an organ of prehension. In man, the radius is shorter than the humerus; in the horse, on the contrary, it is longer.
To give an idea of this proportion, we shall employ what is known as the antibrachial index. This index gives the relation which exists between the length of the forearm and that of the humerus; the length of this latter, whatever may be the actual measurement, is represented by a fixed figure, the number 100. A very simple arithmetical operation gives the proportion--
forearm × 100 -------------, the quotient obtained furnishes the index. humerus
The index is less than 100 if the forearm is shorter than the bone of the arm. The index is more than 100 if, on the contrary, the forearm is longer.
In man, the radius is shorter than the humerus; indeed, in adult individuals of the white race the average index is 74.