40 



GENERAL ANA TOMY OF THE SKELETON 



Osteoblasts are met with in the deeper layer of the periosteum, in the endosteum, and in the 

 lacunae. 



Longitudinal Section of Compact Bone (Figs. 6 and 8). We do not see concentric rings, 



as in a transverse section, but rows of lacunse parallel to the course of the Haversian canals and 



these canals appear like half tubes instead of circular spaces. The tubes are seen to branch 



and communicate, so that each separate Haversian canal runs only a short distance. In other 



/ respects the structure closely resembles that of a transverse section. 



,/ Cancellous bone is found in the interior of flat and irregular bones, forming the bulk of the 

 extremities of the long bones. It consists of anastomosing spicules of bone from a meshwork 

 for red marrow. The spicules have a fibrillar structure, and contain lacunae and canaliculi, 

 but no Haversian systems. 



In the epiphyses the spicules are placed, as a rule, at right angles to the planes of the articular 

 surface (the lines of greatest pressure); these are bound together by other spicules that correspond 

 in direction to the planes of the articulation (the lines of greatest tension). Those spicules 

 nearer the marrow cavity are usually heavier and stronger. 



FIG. 7. Cells of red marrow of the guinea-pig, a-d. Myeloplaques. e-i. Marrow cells proper. 

 }-t. Erythroblasts, some in process of division. (Schafer.) 



f Marrow. There are three varieties: red, yellow, and mucoid. ^ 



Red marrow (medulla ossium rubra) is found in the diploe of the cranial bones, in the cancellous 

 tissue of the vertebrae, ribs, and sternum, and in the extremities of the long bones. Red marrow 

 contains much less fat and is less solid than yellow marrow. It consists of a delicate net- 

 work of retiform connective tissue, supporting a dense capillary plexus; some fat; and numer- 

 ous cellular elements. Surrounding the marrow is the endosteum. The cellular elements 

 of red marrow (Figs. 7 and 12) comprise four main groups: (1) Marrow cells, or myelocytes, 

 which are granular protoplasmic masses, capable of ameboid movements, and containing large 

 nuclei. They are not found in normal blood, but are abundant in leukemia. (2) Small 

 nucleated, reddish cells called erythroblasts are found; they resemble the nucleated red cells 

 of the blood of the embryo; eventually by the loss of their nuclei they become normal red 

 blood corpuscles. (3) Xonnucleated red blood corpuscles; and (4) giant cells, containing one 

 or more nuclei the osteoclasts complete the cellular elements. In addition there are a large 

 nujnber of leukocytes, or white blood cells, i. e., polynudear cells, eosinophiles, and basophiles. 



Yellow marrow is found in the shafts of long bones of adults, and differs from the preceding 

 in the presence of a great quantity of fat and a corresponding decrease in the number of cellular 

 elements. 



Gelatinous or mucoid marrow is formed by the absorption of the fat and the cellular 

 elements of yellow marrow, and by the serous infiltration of the intercellular substance. It is 

 produced by starvation, old age, and certain pathological conditions. Neither yellow nor 



icoid marrow are blood-cell forming in function. 



loodvessels of Bone. Small arteries derived from the periosteum enter ypjkmann's canals 

 and^pass to the Haversian canals and ultimately to the marrow. The cancellous tissue is sup- 

 plied by fewer but larger vessels, which are derived from the periosteum, and which often pene- 

 trate the covering of the compact bone and ramify in the cavities of the spongy tissue. 



Thejnarrpw is supplied by a large artery (sometimes more than one) called the nutrient 

 artery /It "enters the bone by the nutrient foramen, which is usually near the centre of vhe 

 shaft, runs in an oblique canal through the compact substance, giving off branches to this 



