I. In the tibialis posticus. 2. In the peroneus longus. (b.) Yellow fibro-cartilage consists of cells with well-marked nuclei and nucleoli placed in a matrix, which is composed almost entirely of fine elastic fibres, and a small and variable quantity of hyaline, intercellular substance is also usually present. It is more flexible and tough than ordinary cartilage, and has but little tendency to ossify. It is found in the following places :— 1. Epiglottis. 2. Cornicula laryngis. 3. The cuneiform cartilages. The cartilages of the external ear essentially belong to the yellow fibro-cartilage group. The cartilage is invested by a perichondrium, which is plentifully supplied with yellow elastic fibres; these penetrating the matrix of the cartilage, intercommunicate with one another, forming fine plexuses, in the meshes of which small cartilage cells are embedded. The cartilage of the Eustachian tube belongs, according to Kölliker, to the group of hyaline cartilages. The hyaline matrix frequently containing fibres, includes groups of oval and rounded cartilage cells. As the cells near the surface they become gradually smaller, and there is here a layer of closely felted nucleated areolar tissue which represents the perichondrium. The cartilaginous substance and the perichondrium, present no distinct line of separation, but pass gradually one into the other; but here and there the vascular tissue of the perichondrium dips down more or less deeply into the substance. of the cartilage (Stricker). Composition of Cartilage. When permanent cartilage is boiled for a long time, it is resolved into chondrin. This substance appears to be a modification of gelatine, but differs from it in that it is precipitated by acetic acid, the precipitate being soluble in potassium ferro- and ferricyanide, which distinguishes it from albumen. It is also precipitated by dilute mineral acids, and by the salts of aluminium, iron, lead, silver, and copper, whilst its solutions are only rendered slightly turbid by tannic acid. Gelatin is transparent and colourless, and destitute of taste and smell. Solutions of it are precipitated by alcohol and ether, but not by acids, with the exception of tannic acid. Chlorine water and corrosive sublimate precipitate it, but other salts of mercury and those of silver have no such effect upon it. Elastin is the base of elastic tissue, and may be obtained pure from the ligamentum nuchæ of animals by boiling it successively with alcohol, ether, water, concentrated acetic acid, dilute solutions of potash, and hydrochloric acid (Carpenter). CHAPTER IV. Microscopical Appearances and minute Structure of Bone-Physical Properties and Chemical Composition-The Growth of Bone and its Mode of Development in Cartilage and Membrane. BONE. Microscopical Appearances and Structure. If a transverse section of bone is examined under the microscope, it is seen to consist of a number of circular systems, each possessing a central aperture, arranged around which are little irregular spaces with small canals extending from them in all directions. The spaces (P. I. Fig. 1) are called lacunæ (P. I. Fig. 1, 6), and lodge the bonecells, and the channels leading out from them are called canaliculi, and lodge the processes of the bone-cells in the recent state. The canaliculi of the separate lacunæ communicate with one another, and also with the central aperture. If a longitudinal section of bone is now observed, it will be seen that the central apertures in the circular 500 systems are the cut ends of canals, which, running lengthways through the bone, communicate with each other by lateral branches, and are called Haversian canals (P. I. Fig. I, a). They are about th of an inch in diameter, and contain a small artery derived in some instances from the periosteal vessels, and in others from the vessels of the endosteum. The lacunæ and canaliculi absorbing nutrient material from them convey it to the ultimate parts of the bone. The Haversian canals are surrounded by a variable number of concentric lines, from five to ten, or more, called the Haversian lamella, and between these lamellæ are the lacunæ and canaliculi. The Haversian canal, together with the concentric lamellæ, and lacunæ, and canaliculi, constitute what is known as an Haversian system. Every lacuna is occupied by a nucleated mass of protoplasm, the bone-cell or osteoblast which sends its processes into the canaliculi and presides over the nutrition of the osseous substance. Beside the lamellæ mentioned above as being concentric with the Haversian canals, there are other lamellæ which are parallel with the |