Very good specimens of fatty tissue may be obtained by cutting off a small piece of subcutaneous cellular tissue of the calf, placing it in a drop of water on the slide, slightly teazing it out, and then gently applying the covering glass in the usual manner.

Fine sections may also be made from the fat of cold roast beef by cutting it with a razor, selecting the soft surface fat in preference to the suet, which is not good for the purpose.

Isolated fat cells are best obtained from marrow, a minute portion of which should be placed in a drop of water on the slide, and teazed out carefully so as not to injure the delicate cells; the thin glass cover must be very lightly applied, and the specimen examined with a high power.

If too much pressure has been used, the vesicles will be ruptured, and then only masses of oil and numerous granules will be visible.

CARTILAGE.

CARTILAGE or gristle, as it is commonly termed, is a dense nonvascular structure, of a pearly white or slightly yellow colour, distributed with tolerable uniformity throughout the body whereever a certain solidity combined with elasticity is necessary.

Thus, it exists in the anterior walls of the thorax, in the trachea, larynx, and bronchial tubes, in the nose, ears, eustachian tubes, and eyelids. It covers the ends of bones where they are in apposition to form joints, and thus lessens the effects of concussion, and in the embryo it forms the entire skeleton, a considerable part of which remains cartilaginous for some time after birth.

Cartilage is covered by a dense fibrous membrane termed perichondrium, excepting that kind known as articular cartilage, which receives a layer of epithelium from the synovial membrane extending over the whole surface of the joint in the fœtus, but in the adult confined to the circumferent margin, in consequence of the central wear which takes place as soon as the parts are subjected to friction during the movements of the animal's limbs.

Cartilage is distinguished as permanent when it continues unchanged, either covering the ends of bones (articular cartilage), or forming the walls of cavities (membraniform cartilage); and temporary when it is subject to the process of ossification, during the development of the animal textures.

Besides the ordinary kind of cartilage temporary and permanent, there are two modifications of the tissue existing in certain parts of the body, viz. cellular cartilage, which is composed of cells lying close together in a mesh formed of fine fibres; and fibrocartilage, in which the cartilage cells are distributed in a matrix everywhere pervaded by fibrous tissue, either of the white or yellow variety.

For the microscopic examination of ordinary cartilage the head of the humerus of an adult animal should be taken; that from an ox may be easily obtained from the butcher. Fine sections should be made with a razor near the surface and parallel to it; the specimen must next be transferred to a drop of water in the centre of the glass slide, covered in the usual way with a piece

of thin glass, and viewed first with a low, and afterwards with a

high power.

The illustration will convey a general idea of what the observer should see, but it is necessary to notice that the form and arrangement of the cells will be modified by the direction and depth of the section.

Examined with a low power, ordinary cartilage looks like a homogeneous membrane studded over with numerous round, oval, oblong, semilunar, and irregularly shaped corpuscles; under a high power the membrane or matrix loses its homogeneous character, and appears to be either finely granular or faintly striated.

In articular cartilage the matrix is granular, the cells or corpuscles are collected in variously-sized groups of two to six, lying in little hollows, or lacunæ, which the cells occasionally do not completely fill. A membrane is said to line the lacunæ, and to be rendered visible by boiling the cartilage for some hours, and afterwards treating it with acetic acid. This view makes the cartilage cell homologous with the primordial utricle.

The cells sometimes contain granular matter, and occasionally fat-globules. They also possess distinct nuclei, varying from 4000 to 2400 of an inch in diameter, and sometimes also nucleoli. Parent cells will be observed, enclosing several young ones, as the cells multiply by the endogenous process, already explained.

In sections taken near the surface of the joint, the groups of

corpuscles are flattened and run parallel, but in deeper sections, that is to say, nearer the bone, the groups are oblong and narrow, sometimes appearing like strings of beads.

The variety of permanent cartilage attached to the ribs differs from articular cartilage in several particulars. In the first place, the corpuscles are very much larger, have better defined nuclei, and are more regularly grouped together. In the second place, rib cartilage has a tendency to become fibrous, more particularly as the subject advances in years.

These peculiarities are beautifully shown in the illustration, taken from a specimen of rib cartilage from an aged subject.

Cartilage of rib from a man seventy-six years old, showing the develop-
ment of fibrous structure in the matrix. In several portions of the speci-
men two or three generations of cells are seen inclosed in a parent cell
wall.-High power.

Temporary cartilage may be readily distinguished from permanent, by the absence of the groups of corpuscles, the cells being distributed at nearly equal distances apart in the intercellular substance. The cells are mostly round or oval, and the nuclei are granular.

Specimens for examination may be obtained from any very young animal in whom the ossific process is not complete.

The humerus or femur of a calf will furnish good examples. The peculiarities of temporary cartilage, however, are best studied during the investigation of the process of ossification, which will shortly be considered.

Cellular Cartilage. This variety of cartilaginous structure is found in the ears of all small animals from the size of the rat downwards, and even in the tips of the ears of larger animals, such as the rabbit and dog.

In order to examine cellular cartilage, the ear of a mouse should be taken, and first dried until sufficiently firm to be cut with a razor; fine slices may then be shaved off transversely, and placed in a drop of water on the glass slide, covered with the piece of thin glass in the usual way, and examined with a high power.

The drawing indicates the appearance which the specimen should present if properly prepared.

The cells are large, and appear to be packed closely together in cavities in the matrix, but, when minutely examined, it will be found that the corpuscles do not impinge upon each other, but are surrounded by clearly defined fibres, so arranged as to form a very beautiful and perfect network, in the meshes of which the cells are placed.

In many parts of the specimen empty spaces will be noticed,