LESSON XII.

THE CONNECTIVE TISSUES (continued).

COSTAL CARTILAGE. FIBRO-CARTILAGE.

1. MAKE transverse and tangential sections of a rib-cartilage, stain them with magenta, and mount in dilute glycerine, cementing at once. Sketch a part of a transverse section under a low power and a cell-group from one of the tangential sections under a high power. Notice especially the arrangement of the cells, somewhat concentric near the surface but radial near the centre. The costal cartilages are often ossified near the middle.

2. Make sections of the cartilage of the external ear. Mount in dilute glycerine faintly coloured with magenta. If from the ox, notice the very large reticulating elastic fibres in the matrix. Notice also the isolated

granules of elastin, and around the cartilage-cells the area of clear groundsubstance. Draw a small portion of the section.

3. Mount a section of the epiglottis in the same way. Notice the closer network of much finer fibres.

4. Cut sections of white fibro-cartilage (intervertebral disk), and stain them with dilute magenta. Mount in dilute glycerine. Observe the wavy fibres in the matrix and the cartilage-cells lying in clear areas often concentrically striated. Look for branched cartilage-cells. Sketch three or four cells and the adjoining fibrous matrix.

Costal cartilage.-In the costal cartilages the matrix is not always so clear as in the matrix of the joints, for it often happens that fibres

FIG. 54.-SECTION OF RIB-CARTILAGE, SHOWING TWO CELL-GROUPS IN A

SOMEWHAT FIBROUS-LOOKING MATRIX.

become developed in it. The cells are generally larger and more angular than those of articular cartilage, and collected into larger groups

(fig. 54). Near the circumference, and under the perichondrium or fibrous covering of the cartilage, they are flattened and parallel to the surface, but in the deeper parts they have a more irregular or a radiated arrangement. They frequently contain fat. The cartilages of the larynx and windpipe and of the nose resemble on the whole the costal cartilages, but the study of them may be deferred until the organs where they occur are dealt with.

Elastic or yellow fibro-cartilage occurs in only a few situations These are, the cartilage of the external ear and that of the Eustachian tube, the epiglottis and cartilages of Santorini of the larynx, and in some animals, e.g. the ox, the upper third of the arytenoids. The matrix is everywhere pervaded with well-defined branching fibres, which unite with one another to form a close network (fig. 55). These

fibres resist the action of acetic acid, and are stained deeply by magenta; they are evidently elastic fibres. In the ox they are very large, but smaller in man, especially in the cartilage of the epiglottis (fig. 56)They appear to be developed by the deposition of granules of elastin in the matrix, which at first lie singly, but afterwards become joined to form the fibres.

White fibro-cartilage is found wherever great strength combined with a certain amount of rigidity is required: thus we frequently find fibro-cartilage joining bones together, as in the case of the intervertebral disks and other symphyses. Fibro-cartilage is frequently employed to line grooves in which tendons run, and may also be found in the tendons themselves. It is also employed to deepen cup-shaped articular surfaces; and in the case of the interarticular cartilages, such as those of the knee and lower jaw, to allow greater freedom of movement whilst diminishing the liability to dislocation. Under the microscope

white fibro-cartilage looks very like fibrous tissue, but its cells are cartilage-, not tendon-, cells (fig. 57). They are rounded or bluntly angular

FIG. 57.-WHITE FIBRO-CARTILAGE FROM AN INTERVERTEBRAL DISK, HUMAN. (Highly magnified.)

The concentric lines around the cells indicate the limits of deposit of successive capsules. One of the cells has a forked process which extends beyond the hyaline area surrounding the cell, amongst the fibres of the general matrix.

and surrounded by a concentrically striated area of clear cartilagematrix. In some parts of the intervertebral disk many of the cells are branched, and may be looked upon as transitional forms to connectivetissue corpuscles.

LESSON XIII.

BONE AND MARROW.

1. IN thin sections of hard bone made by grinding, observe the Haversian canals, lamellæ, lacunæ, canaliculi, &c. Make a sketch first under a low and afterwards under a high power.

2. With fine forceps strip off a thin shred from a bone which has been decalcified in nitric acid and afterwards kept for some time in dilute alcohol. Mount the shred in water. Observe the fibrous structure of the lamellæ. Look for perforating fibres or the holes from which they have been dragged out. Sketch a small piece of the thin edge of a lamella.

3. Stain with dilute magenta very thin sections of compact bone which has been decalcified in chromic or picric acid, and mount in dilute glycerine, cementing at once. Look for fibres of Sharpey piercing the circumferential lamellæ. The elastic perforating fibres are more darkly stained than the others. Notice the stained nuclei of the bone-corpuscles in the lacunæ. In the thinnest parts of the sections try to make out the blood-vessels and other structures in the Haversian canals.

4. Mount in Canada balsam sections of marrow (from a long bone) stained with hæmatoxylin or borax-carmine.' Observe the fat-cells, the reticular tissue supporting them, the proper marrow-cells in this tissue, &c.

5. Tease in saline solution some of the red marrow from the rib of a recently killed animal. Observe and sketch the proper marrow-cells and look for myeloplaxes and nucleated coloured blood-corpuscles. If examined carefully, amoeboid movements may be detected in the latter and in the marrow-cells.

Bone is a connective tissue in which the ground-substance is impregnated with salts of lime, chiefly phosphate, these salts constituting about two-thirds of the weight of the bone. When bones are macerated this earthy matter prevents the putrefaction of the animal matter. When bones are calcined they lose one-third of their weight, owing to the destruction of the animal matter; when steeped in acid the earthy salts are dissolved and only the animal matter is left. This, like areolar and fibrous tissue, is converted into gelatine by boiling.

Bony tissue is either compact or cancellated. Compact bone is dense like ivory; cancellated is spongy with obvious interstices. The outer layers of all bones are compact, and the inner part is generally cancellated, but the shaft of a long bone is almost entirely made up of

1 See Appendix.

compact substance except along the centre, which is hollow and filled with marrow. The interstices of cancellated bone are also occupied by marrow. Externally bones are covered except at the joints by a vascular fibrous membrane, the periosteum.

True bone is always made up of lamella, and these again are composed of fine fibres lying in a calcified ground-substance. Between the lamellæ are branched cells, the bone-corpuscles, which lie in cellspaces or lacuna. The ramified passages which contain the cell-processes are termed canaliculi.

In cancellated bone the blood-vessels run in the interstices supported by the marrow. In compact bone they are contained in little canalsthe Haversian canals-which everywhere pervade the bone. These canals are about 0.05 mm. (5 inch) in diameter, but some are smaller, others larger than this. Their general direction is longitudinal, i.e. parallel to the long axis of the bone, but they are constantly united by transversely and obliquely running passages. In a section across

FIG. 58.-TRANSVERSE SECTION OF A BONE (ULNA). (Magnified 20 diameters.)

The openings of the Haversian canals are seen encircled by concentric lamellæ. Other lamellæ run parallel with the surface (a).

the shaft of a long bone they are seen as small rounded or irregular holes (fig. 58). When the section has been made by grinding, the holes get filled up with air and débris, and they then look black by

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