Of the 22 cm. of increase in height from the thirty-fourth to the beginning of the fortieth week, 19 cm. is due to the lengthening of the leg from the elbow downwards; but during the last eight weeks (i.e. 40-48) of foetal life there is only an increase of about 9 cm. from the elbow to the ground, while the total increase at the withers is over 20 cm.

At birth the foal of 14-hands parents may be expected to measure 92 cm. at the withers, 62 cm. from the point of the elbow, and 45 cm. from the point of the hock to the ground, the circumference below the knee being 10 cm.

From these figures it appears that during development the actual increase in the length of the limbs is greatest between the twenty-fourth and fortieth weeks. The rate of growth at the withers and from the elbow and hock downwards during development is graphically represented in Table I. Further enquiries may show that during the last eight weeks of foetal life there is a rapid formation of bone, a hardening of tendons and ligaments, and a strengthening of the muscles, so that immediately after birth the foal may, even in times of stress, keep its place in the herd.

Having indicated the periods at which the unborn foal grows most rapidly, I shall now deal with the rate of growth after birth. My observations show that the rate of growth of the foal is decidedly unequal even during the first three months. It might have been assumed either that the increase would be continuous and equal during the first two or three years, or that, rapid at first, it would gradually diminish as the growth power of the bones was lost, From the data already collected it appears that the growth, rapid during the first month, is inconsiderable during the second, but more pronounced during the third, while from the fifth month onwards the rate of growth may be said to diminish gradually, there being, however, ups and downs, related to the shedding of the coat, weaning, or other disturbing causes. In Tables II and III I have indicated the rate of growth that may be expected in a colt (born early in May and weaned in October) that will eventually measure 56 to 58 inches at the withers.

Fig. 566. - Fore-limb (twenty-three weeks), front view, natural size.

Table I. - Showing Kate Of Growth Of A 14-Hands Horse During Development

During the first month the growth at the withers is 11.4 cm., during the second 3 cm., and during the third 10 cm. Throughout the second three months (4 to 6 inclusive) the growth at the withers is about the same as during the third month; from the seventh to the ninth month it is 6 cm., from the tenth to the twelfth 5 cm. Hence, during the first year the total increase in height at the withers may be all but 42 cm. (16½ inches). During the entire second year the growth may not exceed 8 cm. (3¼ inches), and during the third year it may amount to less than an inch.

Table II. - Showing The Rate Of Growth Of A 14-Hands Horse During The First Year

After the third year increase in height, if there is any, results almost entirely from a further lengthening of the vertebral spines that form the skeleton of the withers - the length of these spines is intimately related to the size and weight of the head. In the male wapiti, e.g., the spines are long; in the female, owing to the absence of horns, the head is lighter and withers are absent.

Of the increase of height during the first six months 17.7 cm. are due to the growth of the fore-limb from the point of the elbow downwards, but during the second six months the increase of the fore-arm, wrist, and foot is only 3.7 cm., and after the first year the fore-limb from the elbow downwards only increases 1.4 cm. It is worthy of note that almost the entire increase in the length of the fore-limb below the elbow is due to the growth of the radius (fig. 565). The circumference below the knee increases considerably during the first three months, but alters little during the rest of the first year. In the case of the hind-limb there is a gradual increase from the point of the hock during the first nine months, when the maximum length is often reached - the increase is mainly due to a lengthening of the phalanges and the heel process of the os calcis, not to the middle metatarsal.

Table III. - Showing Rate Of Growth In A Horse During The First Three Years

The length of the head gradually increases all through the first year, when the maximum length is nearly attained. The increase is mainly below the orbit, the space between the inner canthus of the eye and the upper margin of the nostril being nearly doubled during the first two years.

It will be noticed from Table III that the girth is all but doubled during the first year.

Why, it may be asked, is the growth in the horse arrested so much sooner than in man? In the horse, as in man, the majority of the long bones consist of a shaft and of two end-pieces (epiphyses) (fig. 56G, shaft, ep., ep.). The increase in the length of the typical long bones takes place at the junction of the shaft with the epiphyses. This zone of growth is a source of weakness, and the sooner the terminal pieces - which by their free ends enter into the joints - firmly coalesce with the shaft the better. In the horse this fusion takes place at a comparatively early stage, and when it has been once effected all further increase in length becomes impossible.

The question may now be asked: Can any practical use be made of all this information as to the rate of growth in the horse?