PATHOLOGY 367 lage- callus opposite the fracture (from Paget) ; ft, embryonic membrane covering the bone, or the periosteum, and the marrow. During the growing period these two tissues retain pronounced embryonic characters, and at all times they take on a formative action readily. However unlikely an object, then, a bone may seem for repair, it has within and around it the materials for a tolerably direct renewal of osseous substance. The most orderly or intelligible form of the re- parative process is that seen in animals. A long bone, such as the tibia or shin-bone, after having been broken and carefully set, presents an appearance such as is drawn in the figure (iig. 16, a). Opposite the line of fracture there is a fusiform thickening all round the bone, which is bulky and carti laginous for a time, and afterwards becomes greatly reduced in extent, and, at the same time, osseous in its structure. It is called the callus. It will be convenient to de scribe the details of this process of repair from actual specimens of the tibia of a young frog which was found undergoing repair after fracture. The tibia, when cleared of the muscles, was found to have a spindle-like enlargement about its middle of the size and shape of an out (tig. 16, b) r and of a whitish colour. It was easily cut j np into sections passing through its whole length as well as through the projecting ends of the spindle representing the normal shaft of the bone (fig. 17). The bulk of this fusiform enlargement is made up of cartilage developed between the upraised periosteum and the dense substance of the bone. But there is another and independ ent new -formed mass projecting from the canal of the bone, and clearly marked off from the wide extent of cartilage around it, this is the direct osseous formation from Fio. 16. -a, broken tibia of the marrow. The cartilage has been pro- duced from the periosteum, each spindle- cell of the latter altering its form and developing a disproportionate amount of ^f S*^ cell-substance, which becomes the hyaline cartilage covering a frac- matrix of the cartilage, while the nucleus ture. of the original cell, generally excavated or reduced to a crescentic shape, remains as the cartilage -cell. From this cartilage, again, bone is formed very much as it is formed from the ce 1 1 tral rod of cartilage in the fietal bone, and it also resembles the latter in being formed only to lie reabsorbed. In these preparations from the frog, nar row spiculaj of bone may be seen start ing from the thin end of the spindle and spreading over the surface of the cartilaginous cal lus. In the deeper strata of the latter, and still at the thin end of the spindle, the cartilage -cells group themselves round the walls of alveolar spaces, as in the ossification of epiphysial carti lage, and that is doubtless the pro cess which extends throughout the whole mass of car- tih"p Meimvliilo Flr " 17 - Section through broken tibia of a young frog, upper fragment, a, ensheathing callus (cartilage) there has arisen a between periosteum and shaft ; 6, Intermediate callus fungus-like protril- (bone), growing from the cells and vessels of the sion of new bone -row. from the medullary canal of the bone ; it lines the inner walls of the medullary cavity for a short distance up from the line of a -.-- fracture, and projects for a greater distance into the midst of the cartilaginous callus. This centre of ossification is intimately connected with the blood-vessels of the marrow ; they form the framework of the osseous growth, the embryonic marrow -cells (themselves the lineal descendants of cartilage -cells) becoming the osteoblasts or future bone -corpuscles. The whole of the new growth of bone is ultimately moulded into a more compact form ; but the seat of an old fracture will always retain a certain roughness of exterior, and a certain want of regularity in its Haversian systems. The repair of bone in man is not altogether the same as in animals ; the ensheathing cartilage is not usually found except in broken ribs, and the uniting osseous substance corresponds mostly to that part of the new bone (in the preparation from the frog) which issues from the medullary cavity in association with the blood-vessels of the marrow. The callus in man is accordingly said to be chiefly "intermediate" or between the broken ends, and partly also " interior," or extending into the medullary canal ; and it is naturally permanent and not subject to removal like the "ensheathing" callus developed from cartilage. But the sources of new bone in man depend upon the amount of displacement of the broken ends ; if the displacement be very considerable, the connective tissues around may be drawn upon for bone -forming materials, their cells becoming embryonic in form and ultimately osteoblasts. Comparing the repair of a bone with the repair of soft parts, the former is much more direct ; the osteoblastic tendency or memory is strong in the tissues within and around a bone, above all in the periosteum and in the young or red marrow ; and true osseous union is readily effected except in such fractures as the neck of the thigh-bone and the knee-cap, where the union is often merely ligamentons or fibrous. In the "green- stick " fractures of children the periosteum is still a succulent layer engaged in the natural growth of the bones, and there is reason to suppose that it is the chief source of whatever reparative materials may be needed. E,epair of Nerves and Muscles. When a nerve, such as the ulnar, is divided by a cut near the wrist, sensibility is lost over the area of skin to which the nerve is distributed, and, under ordinary cir cumstances, it is restored in about three weeks. The severed ends of the nerve are joined by a band of tissue, which has been proved by examination of it at i various stages of the reparative process in animals to be at first composed of embryonic spindle-cells arranged in the line of the nerve-bundles (fig. 18) ; these cells are derived from the nuclei of the neurilemma, they pass through the ori ginal embryonic phases, and ultimately become more or less perfect nerve-tubes Fic.is. Repaired nerve(frog) filling the gap in the divided nerve, a ten weeks after section; gap which may be a quarter or half an ^f^r^i,lT^ "(l&om inch in length. In muscle, also, a cor- Billroth, after Hjelt.) Musrle- responding process is described ; but the repair, repair of a ruptured muscle such as the rectus extensor of the thigh is commonly fibrous only, and the gap can be felt even through the skin. 1 4. ERRORS OF EMBRYOLOGICAL GROWTH IN CERTAIN TISSUES MESOBLASTIC TUMOURS. No chapter or section treating of tumours as a whole can Tumour, be homogeneous ; and, in order to preserve the develop mental or evolutional order already sketched, it will be convenient to consider here only a part of the morbid processes which result in tumours, leaving the rest to be introduced at appropriate points in the sequel. The dis advantage of applying the developmental or embryological idea to all tumours whatsoever comes out in the tumour- hypothesis of Cohnheim. According to that hypothesis, the tumours of the body are clue to the awakened growth of small centres or foci of embryonic tissue which had 1 Literature. Paget, Lectures on Surgical Pathology, 4th ed., Loud., 1876 ; Darwin, Animals and Plants under Domestication, vol. ii. chap, xxvii., newed., Lond., 1882; Billroth, Ueber die Enticickelung dcr Blutgefasse, &c., Berlin, 1856 ; Id., in Beitrage zur pat hoi. Histol., Berlin. 1858, and in his Allgem. C hirurg. Pathol. (Engl. transl.); Ziegler, Untcrstich. liber pathol. Bindegeicebs- und Gefassneubildung, Wiirzburg, 1876, and in his Pathol. Anat. und Pathogenese, Jena, 1880-84 (Engl. transl.) ; Rindfleisch, Lehrbiich der pathol. Geu-ebelehre (Engl. transl., 1872-73); Golding Bird, "Constructive Inflammation and Ulcers," in Guy s Hosp. Reports, vol. xxiv., 1879, p. 525.
