PASSIVE ORGANS OF LOCOMOTION 431 



bones and gray or gelatiniform in others. It contains certain peculiar 

 cells and nuclei, with amorphous matter, adipose vesicles, connective tis- 

 sue, bloodvessels and nerves, with little bodies, called myelocytes, and 

 free nuclei. These are found in greater or less number in the bones at 

 all ages, but they are more abundant in proportion as the amorphous 

 matter and fat-cells are deficient. The myelocytes resemble the 

 leucocytes of the blood but are a little larger and each has a vesicular 

 nucleus. They are capable of ameboid movements. In addition, the 

 marrow- contains erythroblasts, acidophile cells, large basophile cells, 

 leucocytes, and erythrocytes. The erythroblasts multiply by karyoki- 

 nesis, lose their nuclei and become red blood-corpuscles. The uses 

 and destination of the other varieties of cells have not been definitely 

 ascertained (see Plate I). Irregular nucleated patches, described under 

 the name of myeloplaxes, or giant-cells, more abundant in the spongy 

 portions than in the medullary canals, are found applied to the internal 

 surfaces of the bones. They vary in size and form (measuring -y^oo" to 

 2^0 of an inch, or 20 to 100 ft in diameter), are finely granular and pre- 

 sent two to twenty or thirty nuclei. The nuclei are clear and ovoid 

 and usually contain a distinct nucleolus. The myeloplaxes are ren- 

 dered pale by acetic acid and the nuclei are then brought distinctly 

 into view. They are especially abundant in the red marrow. 



In addition to the anatomical elements just described, the marrow 

 contains a few delicate bundles of connective tissue, most of which 

 accompany the bloodvessels. In the foetus the adipose vesicles are few 

 or may be absent; but in the adult they are quite abundant, and in 

 some bones they seem to constitute the whole mass of the marrow. 

 They do not differ materially from the fat-cells in other situations. 

 Holding these different structures together, is a variable quantity of 

 semitransparent, amorphous or slightly granular matter. 



The nutrient arteries of the bones send branches to the marrow, 

 usually two in number for the long bones, which are distributed between 

 the various anatomical elements and finally surround the fatty lobules 

 and the fat vesicles with a delicate capillary plexus. The veins cor- 

 respond to the arteries in their distribution. The nerves follow the 

 arteries and are lost when these vessels no longer present a muscular 

 coat. Nothing is known of the presence of lymphatics in any part 

 of the bones or in the periosteum. 



The chief physiological interest connected with the marrow of the 

 bones is in its relations to the formation of blood-corpuscles. This 

 question has already been discussed in connection with the development 

 of the corpuscular elements of the blood. 



Periosteum. In most of the bones the periosteum presents a single 



