In ordinary growth of tissue, whether normal or pathological, the blood-vessels strictly follow the growing tissue, and are formed according to its needs. But if, from some accidental circumstance or by artificial interference, the blood-supply be greatly increased, then excessive growth may result. If the spur of the cock be removed from the leg and successfully transplanted into the comb, it will grow with excessive vigour, forming a prominent horn-like structure. Here the increased supply of blood, the comb having a much more active circulation than the leg, induces an excessive growth of the epidermis forming the spur.

The observations of Bizzozero and his pupils are of interest in this relation. By experiments conducted in rabbits he found that where a process of cellular new-formation by karyomitosis was going on, this was not stopped but only rendered less active by starving the animal. On the other hand the process may be greatly accelerated by increasing the blood-supply and elevating the temperature of a part. Thus in a growing rabbit the two ears may be made to grow at very different rates by keeping the one at a temperature of 37° C. and the other at 12° C, the difference depending essentially on differences in the blood-supply. In processes of repair and regeneration such as the healing of wounds and of fractures, the process was similarly accelerated by increase of blood-supply. When a wound was made in the ears and the one was rendered hyperremic by extirpating the cervical sympathetic ganglion, the process of repair was more rapid in the hyperffimic one, and karyomitosis was more vigorous. Similarly in fractures of the limbs the processes of repair and regeneration were more rapid when the limb was kept at a higher temperature.

In human pathology we have numerous instances of hypertrophy from increased activity of the circulation. We see it in the neighbourhood of inflammations. Determination of blood exists outside the immediately inflamed area, and if this is prolonged it may lead to overgrowth of the tissues. In this way we may account for the excessive growth of hair sometimes seen in the neighbourhood of ulcers, near diseased joints, and at the ends of stumps which have remained long inflamed.

A very striking instance of hypertrophy of this kind is sometimes afforded in bones. In the neighbourhood of inflamed joints the surface of the bones is often nodulated, and the bones greatly thickened by new-formation under the periosteum. Again, if a boy has a necrosis of the femur, the whole bone may be more richly supplied with blood, and the normal growth accelerated. The necrosis may be recovered from, and the person be left with a permanently elongated femur which may be as much as two inches longer than the other, and this may lead to considerable lameness. The tibia is differently situated to the femur. Its two extremities are tied to the ends of the fibula by firm ligaments, and so the bone cannot freely elongate. If overgrowth occurs the bone must curve so as to accommodate itself. An example of this is shown in the accompanying Figure, from a preparation in St. Bartholomew's Hospital Museum (quoted by Paget), in which the bone, measured over its curve, was two inches longer than the healthy one (Fig. 68).