Others, being of a more loose and spongy texture, have been called cancellated. These are commonly formed by the bones of the extremities, and often attain a very considerable size; they grow from the periosteum, or from the outer surface of the bone, and are then covered by an expansion of this membrane. Sometimes they adhere by a narrow neck, and expand into a bulbous form, so that they can be very readily removed by operation, and are very apt to be broken off by external injury. Others have a broad and firmer attachment, and are of an irregular shape, often projecting in the form of a large spicula, and at other times assuming a somewhat stalactical appearance. Such frequently prove the source of much inconvenience, by impeding the motions of the muscles, or disturbing the functions of any adjoining organ. They possess no malignant disposition, but are under the same laws, though perhaps in a less degree, with their parent trunk. On making sections of exostoses, and of the bones from which they arise, some appear to be mere enlargements or processes of the parent bone, the cancellated tissue extending itself so as to form the interior of the exostoses, whilst the exterior resembles a proportionate extension of the outer lamina. Others are evidently formed by the deposition of osseous matter externally to the outer lamina, many being dense and compact throughout, others containing an internal cancellated structure, but which is not continuous with that of the bone, as it is separated by the natural outer lamina. Their formation appears similar to that of the fœtal bones: a glutinous matter is effused, becomes dense, and is converted into cartilage; bloodvessels shoot into it, ramify throughout its whole structure in a radiated form, and deposit osseous matter. This deposit increases, and extends from a central nucleus towards the circumference; the cartilage is in part absorbed, and the new structure becomes osseous, and similar to the original tissue by the vessels of which it was formed. These tumours, even when they have attained a large size, contain a mixture of bone and cartilage, covered by a dense fibrous investment. The bony matter is arranged in spicula, adhering to the surface of the shaft or head of the bone, and projecting into the morbid mass: the spaces are filled up by a cartilaginous substance. The growth is sometimes rapid, and the tumour soon becomes troublesome from its immense size.