340 THE TISSUES. 



which occurs in their epiphyses, and in the short and flat bones, the 

 sternum, and the bodies of the vertebras. These two forms are also 

 quite different in chemical composition. While the former is made 

 up (in the bones of the ox, according to Berzelius) of 96 per cent. 

 of fat, 1 of areolar tissue, arid 8 of fluid with extractive matter, the 

 latter (in the diploe) contains 75 per cent, of water, the rest (25 per 

 cent.) being made up of solid matters, albumen, fibrine, and ex- 

 tractive matter, similar to those of muscle, with merely traces of fat. 

 In its structure, marrow presents, besides vessels and nerves, areo- 

 lar tissue, fat-cells, free fat, and a yellowish fluid ; and, lastly, pecu- 

 liar minute cells marrow-cells. 



1. The areolar tissue inclosing the marrow of the shaft of long 

 bones is of a firm, consistency, but is improperly described as an 

 endosteum (internal periosteum), since it cannot be separated as a 

 distinct structure. It also penetrates the marrow of the long bones, 

 and supports the vessels and nerves ; while it does not enter at all 

 into the marrow of the cancelli, except in case of the larger masses 

 of it. 



2. Fat-cells , ^J^ to -gfa of an inch in diameter, and often with a 

 distinct nucleus, occur in both forms of marrow; more abundantly, 

 however, in the yellow, dense form, and generally not aggregated 

 into lobules. In the red variety they are far less numerous, and 

 often isolated even; and hence the small quantity of fat in the 

 diploe. (Berzelius.) In dropsical marrow, these cells are frequently 

 only half filled with fat, or with but one or more globules ; con- 

 taining, besides, a large quantity of serum (p. 307). In hypersemia 

 of the bones, they are sometimes diminished in size, and are also 

 elongated and fusiform. 



3. Free fat-globules, and a clear or yellowish fluid, are often met 

 with in the softer kinds of marrow. The former may possibly have 

 been derived from cells which no longer exist. 



Lastly, the marrow-cells occur in the red or the reddish marrow, 

 but never in the yellow. They exist normally in the vertebrae, the 

 cranial bones, the sternum, and the ribs ; and in the upper maxil- 

 lary bone, also, where they have been mistaken for cancer-cells. 

 They also occur in the hyperaemiated red marrow of the articular 

 extremities of the cylindrical bones ; but are normally absent in all 

 the bones of the extremities, and in the scapulas and the coxal bones. 

 They precisely resemble the cells of the young medulla (p. 355). 



Use of the Marrow. The cavities of the bones in man must be 



