ADDITIONAL EXERCISE.

13. Mucous Tissue. Many branched cells are seen in a T.S. of the tail of a tadpole, young triton, or salamander, hardened in 1 per cent. osmic acid and cut in paraffin.

It is also to be found under the skin of the flanks in frogs at the breeding It gives good preparations when stained with methyl-violet-5B

season.

(S. Mayer).

LESSON XIII.

BONE, OSSEOUS TISSUE, &c.

THE essential elements of osseous tissue, of which bone consists, are a calcified fibrous matrix or ground-substance, with cells or bone-corpuscles embedded in it; the latter are lodged in spaces called lacunce. A bone, however, is a complex organ. The following scheme may facilitate the comprehension of its minute structure :

BONE.

In a longitudinal section of a long bone, observe with the naked eye

Periosteum covering the bone.

Compact or dense bone, the substantia dura (with Haversian
canals).

Cancellated or spongy bone, the substantia spongiosa (with
Haversian spaces and cancelli).

Medullary cavity with marrow.

Histologically dry compact bone shows

Lamella

Peripheric or circumferential.
Haversian or concentric.

Intermediary, interstitial or ground.
Perimedullary.

Sharpey's perforating fibres white.

Dry bone.

Haversian canal.

yellow.

HAVERSIAN SYSTEM.

1. External layer, fibrous, with the larger blood

vessels.

Periosteum 2. Internal or osteogenic layer, with finer blood-
vessels, numerous elastic fibres, and osteo-
blasts, and sometimes osteoclasts.

Osteoclasts or myeloplaxes of Robin.
Blood-vessels, nerves, and lymphatics.
Development

1. Endochondral in cartilage.
2. Intra-membranous or periosteal.

1. T.S. of Dense or Compact Bone. It is better to buy a prepared transverse section of the shaft of a small, dry, long bone. In it most of the smaller spaces, being filled with air, appear black.

(a.) (L) Observe the medullary cavity, bounded by a ring of bone, less dense internally.

(b.) Sections of tubes which appear round, or oval-Haversian canals surrounded by concentric lamellæ (fig. 145, c), and between these lamellæ are lacunæ (e), with fine channels passing from them-the canaliculi. Some lamellæ are arranged parallel to the circumference of the bonethe peripheric (a)-while others of larger radius are incomplete, and jammed in between the Haversian system: they are intermediary (). Around the medullary canal are the perimedullary lamellæ ().

(c.) (H) Observe the shape of the lacunæ-flattened branched spaces with their numerous wavy branching canaliculi, and how adjoining canaliculi anasto

mose by traversing the lamellæ FIG. 145.-T.S. Human Metacarpal Bone.

(figs. 145, 147, e). At the outer part of each Haversian system, some of the canaliculi of the

a. Peripheric lamella; b. Perimedullary lamellæ ; C. Haversian canals surrounded by their Haversian lamellæ ; d. Intermediary lamella; e. Lacunæ, X 20.

outermost row of lacunæ will be found to form loops and open into the lacunæ from which they arose these are recurrent canaliculi (fig. 147, a). The canalicular system is for the distribution of lymph to all the parts of the calcified fibrous matrix. Notice also that the intermediary lamellæ are parts of circles with a much

II.

larger radius than those of the Haversian system. Each Haversian canal, with its system of lamellæ, lacunæ, and canaliculi, forms a Haversian System. The greatest diameter of the lacunæ is

about 14 μ (1800 inch).

(d.) The lamellæ in a Haversian system on transverse section appear as thin concentric bands, a clear transparent one alternating with one which looks more granular. These are not due to different kinds of lamellæ,

FIG. 146.-T.S. The Shaft of a Human Femur. Haversian canals; s. Haversian lamella; si. In- but in the clear ones one termediary lamellæ, x 40. looks upon the long axis of the fibrils composing the lamella, and in the others upon the ends of the fibrils.

2. L.S. of Dense Bone, prepared in the same way.

(a.) (L) Observe Haversian canals running chiefly in the long axis of the section, with here and there oblique, short, junction canals. Near the surfaces some open externally, and others communicate with the medullary cavity. The lamellæ of any Haversian system run parallel to its own Haversian canal. In the system of canals-each 20-100 μ wide-the canals frequently divide dichotomously, and ultimately form a network in the compact bony substance.

(b.) If the section be near the surface of the bone, so as to include the peripheric lamellæ, canals for blood-vessels, perforating the lamellæ and not surrounded by lamellæ as in the Haversian systems, may be seen. They are called Volkmann's canals, and contain the perforating vessels. They are connected with the Haversian canals proper. They are well seen in sections of the femur of a guinea-pig, but, unlike Haversian canals, they are not surrounded by Haversian or concentric lamellæ.

(c.) (H) Observe the flattened oval lacunæ with their canaliculi;

If a

their arrangement, as well as that of the lamella (fig. 148). canal be viewed carefully many fine dots will be seen in it. These are the openings of the canaliculi.

3. T.S. Decalcified Shaft of a Bone.-The bone, cut into short lengths, must be decalcified in picric acid or chromic and nitric fluid, with the precautions laid down at p. 38. When sufficiently

FIG. 147.-T.S. Shaft of Human Femur. H. Haversian canals; c. Lacunæ with bone-corpuscles; a. Lacunæ with recurrent canaliculi; s. Intermediary lamellæ with Sharpey's fibres, h; p. Large fibres of Sharpey in intermediary lamella; 7. Confluent lacunæ. These Ranvier supposes are bone-corpuscles and lacunæ undergoing atrophy, X 300.

FIG. 148.-L.S. Dense dry bone, X 40.

soft-ascertained by pricking it with a pin-it is hardened in alcohol in the usual way. Sections are best made by freezing.

(i.) Stain a section in picro-carmine and mount it in glycerinejelly. Glycerine or Farrant's solution tends to make the preparation rather too transparent.

(ii.) Place some sections in 1 per cent. osmic acid for twenty-four hours and mount them in glycerine-jelly.

(a.) (L) Observe the periosteum (fig. 149), embracing and adherent to the bone. In the bone itself the lacunæ, and especially the canaliculi, are no longer black, and are not so visible as in the non-decalcified bone. Each lacuna contains a highly refractive, branched, nucleated, and stained corpuscle.

Bone-Corpuscles.-Observe their arrangement following that of

the lamellæ, but the latter are not so distinct as in dry bone. Several lamellæ lie between two consecutive rows of bone-corpuscles. The Haversian canals contain blood-vessels, connective tissue, and other cells, or osteoblasts.

(b.) (H) The periosteum consists of an external layer stained red, and composed chiefly of white fibrous tissue. Attached to it may be found small fragments of striped muscle. The internal layer contains many elastic fibres, and, especially in young bones, there may be seen one layer or more of flattened or cubical cells,

FIG. 149.-T.S. Part of a Human Metacarpal Bone, x 50. h. Haversian space with marrow. Dilute nitric acid.

called osteoblasts (fig. 156, c). The latter may be seen not only under the periosteum, but also passing along with blood-vessels into the Haversian canals.

Fibres may be seen passing from the deep surface of the periosteum into the bone-the perforating fibres of Sharpey.

(c.) Observe the lamellæ, but their outline is not very distinct, while the canaliculi will not be "distinctly visible, being indicated by fine lines traversing the lamellæ. The bone corpuscles are nucleated refractive cells, each lying in a lacuna. In such a

preparation, one cannot make out that they send processes into the canaliculi.

(d.) Select a large Haversian canal and study its contents. Note the presence of an artery and vein with very thin walls, and the cavity lined by osteoblasts (fig. 150, Obl.), and the remainder filled up with medullary tissue.

4. Perforating Fibres. (i.) From a membrane bone of the skull (e.g., the parietal or frontal bone, which has been softened in 2 per cent. hydrochloric acid or in v. Ebner's fluid (p. 37)

.