QUESTIONS.

QUESTIONS FOR CHAPTER I.

1. What is histology?

2. What is a cell?

3. What structures does a typical cell contain?

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4. What are the cell-body? spongioplasm? hyaloplasm? exoplasm?

5. What is the cell-wall?

6. What are the nucleus? nuclear membrane? nuclear network? nuclear matrix?

7. What is the nucleolus?

8. What is the centrosome?

9. Name the properties of the cell.

10. What is meant by metabolism? growth? reproduction? irritability? motion? function?

11. How may a cell divide?

12. What is meant by direct division?

13. Name the synonyms of indirect division.

14. Describe the process of indirect cell-division.

15. What is the origin of all cells?

16. What are the primary germ-layers?

17. What tissues are developed from the epiblast? from the mesoblast? from the hypoblast?

18. What is meant by the parablast? and what tissues are developed from it?

19. Name the four groups of tissues.

20. Name the connective tissues.

21. Describe mucous tissue, white fibrous tissue, tendon, yellow elastic tissue, adipose tissue, retiform tissue, adenoid tissue.

22. Name the varieties of cartilage.

23. What is the perichondrium?

24. Describe hyaline cartilage.

25. What is its matrix? What are its lacunæ? What is its capsule?

26. Describe white fibrous cartilage.

27. Describe yellow elastic cartilage.

28. Describe cellular cartilage.

CHAPTER II.

BONE.

HISTOLOGICALLY, there are two types of bone, the compact and the spongy or cancellated. Covering the surface of bone, except at articular surfaces, we find a membrane, the periosteum, which is analogous to the perichondrium in cartilage, and, like it, consists of two layers—an outer fibrous and an inner or cellular. The outer layer consists principally of white fibrous tissue. The cellular layer contains many more connective-tissue cells, which gradually become more closely aggregated as we proceed toward the osseous surface, leaving no sharply defined line of demarcation between the two periosteal layers. As the bone is developed through the agency of the periosteal cells, they are sometimes called the osteogenetic cells, and the cellular layer, osteogenetic layer of the periosteum.

Compact bone occurs in the shafts of all the long bones, at the inner and outer tables of the flat bones, and as a thin layer around the periphery of the irregular bones. It is not a homogeneous structure, but is composed of thin layers of osseous substance, characteristically arranged, called lamella. At the inner and the outer (medullary and subperiostal) surfaces are a number of lamella of bone arranged concentrically with the periphery and called the circumferential or fundamental lamellæ. Between the inner and outer circumferential lamellæ we find the osseous sub

stance, arranged in systems of concentric layers, each system virtually forming a tube with laminated walls. These tubes, for the most part, run parallel to the long axis of the bone, but at frequent intervals are united by diagonal branches, so that the lumens form a continuous network of canals. These canals communicate with the medullary cavity, and serve to transmit bloodvessels, nerves, and lymphatics, which are supported

Fig. 9.-Transverse section of bone: a, External circumferential lamella; b, Haversian lamella; c, interstitial lamella; d, internal circumferential lamella; e, lacunæ in which lie the bone-cells; f, canaliculi joining above; g, Haversian canals.

by a fine prolongation of the marrow reticulum. These tubes are called the Haversian canals; the strata of their walls, the Haversian or special lamellæ ;` and the canals, together with their bounding lamellæ, contents, and bone-cells (soon to be described), constitute the Haversian system. The more or less triangular spaces between adjacent Haversian systems are occupied

by indefinitely arranged layers of bone, called the interstitial or ground lamellæ. Besides the Haversian canals there is another system of blood-channels. These occur scattered through the circumferential lamellæ, and have received the name of Volkmann's canals. Their blood-vessels communicate both with the surface of the bone and with the Haversian canals. There is no special arrangement of the bone around the Volkmann's canals, as occurs around the Haversian systems, the former being simply spaces between the lamellæ.

Scattered between the individual lamellæ of all three types we find many elongated, thin spaces-the cellspaces, or lacunæ. In the circumferential lamellæ their long axes correspond to the long axis of the bone. In the Haversian lamellæ the lacunæ are slightly curved laterally, to conform to the general concentric annular arrangement. In the interstitial lamellæ there seems to be no special arrangement, but in a general way the long axes of the lacunæ correspond to that of the bone.

Radiating in all directions and connecting the lacunæ are minute canals, called canaliculi, which serve to transmit nutriment to the cells within the lacunæ and communicate with the lymphatics in the canals. While the canaliculi radiating from the lacunæ of a Haversian system intercommunicate very freely, they rarely or never have any direct connection with the adjacent systems.

In dry preparations of bone which have been ground to the requisite degree of thinness for microscopic examination the lacunæ and canaliculi become filled with particles of dirt and debris and appear black.

In sections which have been decalcified, however, the lacunæ are found to contain cells with long protoplasmic prolongations reaching into the canaliculi. These are

the true bone-cells, or bone-corpuscles. If some of the outer lamellæ be forcibly torn off, there will be observed occasional fibers which penetrate the bone vertically to the surface. These fibers are the remains of old periosteum which in the process of bone formation have failed to be displaced by the newly deposited subperiosteal bone. They are called perforating fibers of Sharpey.

Spongy or cancellated bone differs from the compact in that the plates or spicules of which it is composed have no regular arrangement into a system, but unite irregularly to form a honeycombed mass, the cavities of which are filled with marrow. The nutrition of the bone is derived from this marrow, hence there is no need for special blood-vessels. Spongy bone is found at the ends of the shafts of long bones and at the centers of the short and flat bones (the apophyses and diploe). At this place it might be well to call attention to several points in which bone and cartilage are analogous-viz.:

1. They both have a common mesoblastic origin. 2. Both are covered by similar membranes-perichondrium, periosteum.

3. Both are composed of cells embedded in an indifferent matrix.

4. Both have similar functions-supportive framework.

MARROW.

There are two kinds of marrow: the red, which occurs in spongy bone,-principally in the flat bones, the vertebræ, the sternum, and the ribs, and the yellow, which is found principally in the medullary cavities of the extremities. Small prolongations also extend into the larger Haversian canals.