nucleus. The vesicles are supported partly by filaments of areolar tissue, but chiefly by a fine network of capillary blood-vessels.

The fat when first formed is deposited within granular cells of areolar tissue (fig. 45). It appears to be produced by a transformation of

FIG. 44. A FEW CELLS FROM THE MARGIN OF A FAT-LOBULE.

f.g, fat-globule distending a fat-cell; n, nucleus; m, membranous envelope of the fat-cell; cr, bunch of crystals within a fat-cell; c, capillary vessel; v, venule; ct, connectivetissue cell; the fibres of the connective tissue are not represented.

albuminous granules into droplets of fat. As these droplets increase in size they run together into a larger drop, which gradually fills the cell

FIG. 45.-DEPOSITION OF FAT IN CONNECTIVE-TISSUE CELLS.

f, a cell with a few isolated fat-droplets in its protoplasm; f', a cell with a single large and several minute drops; ", fusion of two large drops; g, granular cell, not yet exhibiting any fat-deposition; c t, flat connective-tissue corpuscle; c, c, network of capillaries.

more and more, swelling it out so that the cell-protoplasm eventually appears merely as the envelope of the fat-vesicle.

Fat is found most abundantly in subcutaneous areolar tissue, and under the serous membranes; especially in some parts, as at the back of the peritoneum around the kidneys, under the epicardium, and in the mesentery and omentum. The yellow marrow of the bones is also principally composed of fat. There is no adipose tissue within the cavity of the cranium.

Retiform or reticular tissue (figs. 46, 47) is a variety of connective tissue in which the intercellular or ground substance has mostly disappeared or is replaced by fluid. There are very few or no elastic

FIG. 46.-RETIFORM TISSUE FROM A LYMPHATIC GLAND, FROM A SECTION WHICH HAS BEEN TREATED IN THE MANNER DESCRIBED IN § 5. (Moderately magnified.) tr, a trabeculum of connective tissue; r, r', retiform tissue, with more open meshes at r and denser at '.

fibres in it, and the white fibres and bundles of fibres form a dense network, the meshes of which vary in size, being very small and close in

tr

some parts; more open and like areolar tissue in other parts. In some places where the tissue occurs the fibres are almost everywhere en

wrapped by flattened branched connective-tissue cells, and until these are removed it is not easy to see the fibres.

Lymphoid or adenoid tissue is retiform tissue in which the meshes of the network are largely occupied by lymph-corpuscles. This is by far the most common condition of a retiform tissue, and is met with in the lymphatic glands and allied structures (see Lesson XXII.), and also in the tissue of the alimentary mucous membrane, and in some other situations.

Basement membranes (membranæ propriæ) are homogeneous-looking membranes, which are found forming the surface-layers of connectivetissue expansions in many parts, especially where there is a covering of epithelium, as on mucous membranes, in secreting glands, and elsewhere. They are generally formed of flattened connective-tissue cells joined together to form a membrane; but, in some cases, they are evidently formed not of cells, but of condensed ground-substance, and in others they are of an elastic nature.

Jelly-like connective tissue, although occurring largely in the embryo, is found only in one situation in the adult-viz., forming the vitreous humour of the eye. It seems to be composed mainly of soft ground-substance, with cells scattered here and there through it, and with very few fibres, or none at all. These several varieties of connective tissue will be more fully described in connection with the organs where they occur.

LESSON X.

THE CONNECTIVE TISSUES (continued).

ELASTIC TISSUE, FIBROUS TISSUE, DEVELOPMENT OF CONNECTIVE TISSUE.

1. TEASE out as finely as possible a small shred of elastic tissue (ligamentum nuchæ of the ox or ligamenta subflava of man) in glycerine and water, slightly coloured by magenta. Cover and cement the preparation. Note the large well-defined fibres constantly branching and uniting with one another. Look for transverse markings on the fibres. Measure three or four. Sketch a small part of the network. Note the existence of bundles of white fibres amongst the elastic fibres.

2. Examine a thin transverse section of ligamentum nucha which has been hardened in per cent. solution of bichromate of potash. The section is to be stained with hematoxylin and mounted in Canada balsam by the usual process, or simply in glycerine and water. Observe the grouping of the fibres and their angular shape. Notice also the nuclei of connectivetissue cells amongst the fibres. Sketch one or two groups.

3. Pinch off the end of the tail of a dead mouse or rat, draw out the long silk-like tendons and put them into saline solution. Take two of the longest threads and stretch them along a slide, letting the ends dry firmly to the slide but keeping the middle part moist. Put a piece of hair between them and cover in saline solution. Observe with a high power the fine wavy fibrillation of the tendon. Draw. Now run dilute acetic acid (0.75 per cent.) under the cover-glass, watch the tendons where they are becoming swollen by the acetic acid. Notice the oblong nucleated cells coming into view between the tendon bundles. Sketch three or four cells in a row. Lastly, lift the cover-glass, wash away the acid with distilled water, place a drop of Delafield's hæmatoxylin solution on the tendons, and leave the preparation until it is deeply stained; then wash away the logwood and mount the preparation in faintly acidulated glycerine. Cement the cover-glass with gold size.

4. Take one or two other pieces of tendon, and, after washing them in distilled water, stretch them upon a slide as before, fixing the ends by allowing them to dry on to the slide. Put a drop of nitrate of silver solution (1 per cent.) on the middle of the tendons, and leave it on for five to ten minutes, keeping the preparation in the dark. Then wash off the silver nitrate with distilled water, and expose the slide to direct sunlight. In a very few minutes the silvered part of the tendons will be brown. As soon as this is the case, dehydrate the tendons with alcohol in situ upon the slide, run off the alcohol, and at once put a drop of clove-oil on the preparation. In a minute or two the clove-oil can be replaced by Canada balsam in xylol, and covered.

5. Stain with magenta solution1 a thin section of a tendon which has been hardened in 70 per cent. alcohol. Mount in dilute glycerine and cement the cover-glass at once. Sketch a portion of the section under a low power.

1 See Appendix.

Elastic tissue is a variety of connective tissue in which the elastic fibres preponderate. It is found most characteristically in the ligamentum nuchæ of quadrupeds and the ligamenta subflava of the vertebræ, but the connective tissue of other parts may also have a considerable development of elastic fibres. It occurs also in an almost pure form in the walls of the air-tubes, and uniting the cartilages of the larynx. It also enters largely into the formation of the walls of the blood-vessels, especially the arteries.

In the ligamentum nucha the fibres are very large and angular (fig. 48); they often exhibit cross-markings or even transverse clefts. When dragged asunder, they break sharply across; they constantly branch and unite, so as to form a close network. In transverse section they are seen to be separated into small groups (fig. 49) by intervening white bundles of connective tissue.

Elastic tissue does not always take the form of fibres, but may occur as membranes (as in the blood-vessels). Sometimes the fibres are very small, but their microscopical and chemical characters are always very well marked (see p. 40).

Fibrous tissue is almost wholly made up of bundles of white fibres running in a determinate direction. These again are collected into