(b.) The branching stellate spaces, interfascicular spaces (fig 129, c), between the fasciculi or bundles of fibres. If these spaces contain air, they appear somewhat dark. These spaces can readily be seen as branched dark spaces if a transverse section is madeby means of a knife, not a razor-of a small tendon dried at the ordinary temperature.

(c.) (H) Observe the cut ends of the fibres (1), which appear almost homogeneous, but amongst them here and there may be seen a few dots, which are the transverse sections of elastic fibres,

FIG 130.-L.S. Human Tendon (Tibialis anticus).
SZ. Rows of nuclei of tendon-cells.

and the branched tendonspaces (c), some of them with a nucleated branched cell.

or

2. L.S. Tendon.-Stain a section with logwood, and mount it in balsam. (H) Observe the longitudinal arrangement of the fibres, and between them rows of fusiform tendon - cells, rather the long fusiform nuclei of the tendon cells arranged between the fibres. The other parts of the cell become too transparent to be seen (fig. 130). Sometimes a L.S. of one of the septa may be seen.

3. Fibrils in Tendon (H).-Macerate a tendon from the tail of a rat for twenty-four hours in a saturated solution of picric acid, or for 3-4 hours in baryta water. Tease a small part, and examine it first in

water. Mount it in Farrant's solution. This is apt, however, to render the fibrils too transparent. Perhaps a better method is to place the fine tendons for twenty-four hours in equal parts of I per cent. osmic acid and I per cent. silver nitrate. Mount a teased preparation in Farrant's solution.

(a.) Observe the isolated fibrils (50000 inch in diameter), excessively fine, wavy, and unbranched (fig. 131).

4. Tendon of Rat (Gold Chloride Method).-Kill a rat, cut off its tail, forcibly rupture the tail, when a long leash of fine white

glistening threads or tendons will be obtained.

Prepare the

tendons by one of the gold chloride methods. One of the best methods is the lemon-juice method of Ranvier (p. 79), but the boiled formic acid and gold method also yields excellent results. It is to be remembered that it is not necessary to use gold chloride to demonstrate the tendoncells; this can be done by hæmatoxylin.

With regard to the action of gold chloride, my experience leads me to believe, that in order to see the rows of tendon-cells with their lateral protoplasm expansions, the lemon-juice method is very good; while the old acetic acid method makes the fibres less swollen up, and on teasing they are readily isolated, thus enabling one to see cells either

Rat Isolated by Picric Acid,
X 300.

singly or in rows clasping them. Not un- FIG. 131.-Fibrils of Tendon of frequently isolated tendon-cells are to be seen in the field of the microscope.

Tease a small part of the gold tendon in Farrant's solution. (a.) (H) The fibres are swollen up and transparent, and lying on them are rows of tendon-cells (fig. 132, b, b) stained of a violet Each cell is somewhat oblong with a distinct nucleus, and

tint.

bears a flattened wing-shaped expansion (fig. 133). Along the cells is usually to be seen a stripe or ridge (fig. 133), produced by the cells being compressed between several adjoining fibres. This ridge may be seen to be interrupted in some of the cells. The nuclei of the adjacent cells may be scen to be close together.

(b.) If a side view of the cells is obtained (fig. 132, a), they partially clasp the fibre, but never envelope it completely; in this respect these cells differ from endothelium.

5. T.S. of Gold Tendon (H).-Remove the skin from the tail of a young rat, cut out a piece of the tail a quarter of an inch in length with its tendons, and subject it to the gold chloride process (p. 79). When, after reduction, it has become purple or brownish, decalcify the bone, harden it in alcohol, and make transverse sections. Mount one in balsam.

(a.) Many tissues will be seen, including muscle, nerve, fat, and bone. Neglecting these, observe the small rounded areas at the circumference, the transverse sections of the small tendons, each surrounded by its own sheath of connective tissue (fig. 134, t). In each observe the branched stellate spaces (fig. 134, c), frequently anastomosing with each other. These interfascicular spaces are

purplish in colour; they contain the tendon-cells, and also a purplish deposit due to the gold chloride acting on the lymph which they contain in the fresh condition.

6. Endothelial Sheath of Tendon (L and H)-Silver a leash of the fine tendons from the tail of a rat. Mount a short length of one of them in balsam.

(a) Observe the tendon made up of parallel fibres, and note on their surface a single layer of endothelium. The squames are large, polygonal, and mapped out by "silver lines," but no nuclei are visible.

7. Fresh Tendons and Acid Logwood (H).-Place three or four tendons (rat's) 14 inch long, on a dry slide, and fix their ends with paraffin, so as to keep them extended. Make a solution of acid logwood by adding one part of 1 per cent. glacial acetic acid to three parts of logwood solution. This solution is red. Place a drop of it on a cover-glass, and lay it on the tendons. The acid brings into view rows of narrow, granular, nucleated cells between

the fibres of the tendon, while at the same time the logwood stains them. Instead of acid logwood use picro-carmine or acid hæmatoxylin (p. 69). Displace the dye with water and mount in glycerine. The tendons are purposely taken longer than the breadth of the cover-glass, so that they may remain stretched.

8. Fresh Tendon.-On a black surface, tease in normal saline a small piece of any tendon of a calf. Observe the fibres and fibrils, but no cells are visible. Irrigate the preparation with 2 per cent. acetic acid. The mass becomes clear and transparent to the naked eye, and now under the microscope one sees the fusiform nucleated cells singly or in line in order between the swollen-up fibres. The preparation may be stained with magenta, which brings into clearer view the cells, and any elastic fibres present.

ADDITIONAL EXERCISES.

9. Tendon of Rat (Dogiel's Method).-Very good preparations are obtained by placing the fresh tendons for several days-the longer the better-in Grenacher's alum-carmine (p. 65). This fluid stains but slowly. The cells, however, are stained, and if a tendon be teased, isolated cells, and cells on the fibres are easily seen. It is a good method for showing the relations of the

cells to the fibres.

10. T.S. Tail of Rat (Corrosive Sublimate and Borax-Carmine).-Harden short lengths of the tail of a rat, the skin being first removed, in corrosive sublimate for three hours or so. Remove every trace of the mercuric salt by prolonged washing in alcohol. Stain the tissue in bulk in borax-carmine, and then decalcify it in dilute hydrochloric acid. Make transverse sections, after embedding it by the interstitial method in paraffin. Sections may also be made by freezing, but they are apt to fall asunder. This method also yields beautiful preparations, comparable to those by the gold chloride methods. The transverse sections of the tendons are very characteristic.

11. Dried Rat's Tendons.-A very convenient method is to dry the tendons of a rat's tail, keeping them extended during the process. After drying, they can then be used at any time. By acting on them with dilute acetic acid they swell up slowly, and the rows of cells are thereby revealed. The cells after washing away the acetic acid-can be stained with picrocarmine, and the preparation mounted in dilute glycerine.

12. Cell-Spaces (Saft-Canälchen) in Central Tendon (Silver Method).- Place the central tendon or the whole diaphragm for five minutes in a per cent. silver nitrate. Remove it, and with a camel's-hair pencil brush both surfaces

FIG. 136.-Cell-Spaces in the Central Tendon of the Diaphragm of Rabbit. 7 Lymphatic; s. Cell-spaces. Silver nitrate.

of the tendon to remove the endothelium. Replace it in the silver solution for fifteen minutes. Remove it; wash it in water, and expose it to light to

reduce the silver. One piece may be mounted in balsam; another piece should be stained with acid logwood or picro-lithium carmine (in this case use dilute hydrochloric acid), and mounted in balsam.

(a.) Observe a large number of clear, branched anastomosing spaces, surrounded by brown areas of ground-substance. The former are the cell-spaces and Saft-Canälchen, or juice-canals, and some of the latter may be seen to communicate with the lymphatics (fig. 136).

(b.) In the stained specimen, stained nuclei are seen in the spaces, i.e., the nuclei of the cells which occupy these spaces.

13. Cell-Spaces (Iron Sulphate Method).-Using the fresh central tendon of the diaphragm of a mouse or guinea-pig or rat, place it for a few minutes in I per cent. sulphate of iron. Pencil away the surface endothelium, and leave it in the iron solution for five to seven minutes. Remove it, wash it, and place it in I per cent. ferricyanide of potash, in which it becomes blue. Mount it in Farrant's solution or balsam. In this preparation the cell-spaces and juicecanals are again clear, but the ground-substance is blue.

14. Cell-Spaces in Rat's Tendon. -The fine tendons are placed in silver nitrate (per cent.) for two minutes, and then the epithelium is brushed off by a camel's-hair pencil. Five or six sweeps of the brush usually suffice. The tendons are stained for other ten minutes in silver, washed, and exposed to light in alcohol. Rows of clear, somewhat quadrangular spaces in a brownish matrix are obtained.

LESSON XII.

ADIPOSE, MUCOUS, AND ADENOID TISSUESPIGMENT CELLS.

ADIPOSE TISSUE (FATTY TISSUE).

Adipose Tissue.-A fat-cell consists of a membrane enclosing a globule of oil, which pushes the oval flattened nucleus (surrounded by a small amount of protoplasm) to one side, so that it lies close under the cell-wall. Size, 40 μ to 80 μ (30-30 inch).

1

Fat-cells are arranged in groups, which form lobules, and these again form lobes. Each lobule has an afferent artery, one or two efferent veins, and a dense network of capillaries between the fatcells, each capillary surrounding one or more fat-cells.

It is to be remembered that cells in connective tissue containing fat may have a two-fold origin. Fat may be formed in ordinary connective-tissue cells, but there are other cells of a connectivetissue nature, which seem to be more specifically fat-cells. During development this tissue is formed at certain parts, e.g., in the groin, axilla, and neck, and presents a grayish-yellow appearance in the form of lobules, surrounded by connective-tissue-readily seen in a young animal. The cells at first contain granules. The