THE SKELET.\L SYSTEM 197 



next to the bone and consisting of Ioosel_\' arranged connecti\-e tissue 

 (areolar tissue) with numerous osteoblasts. ■* 



From the periosteum distinct bundles of white fibres, with often 

 some elastic fibres, pierce the outer layers of the bone. These are 

 known as the perforating fibres of Sharpey. When tendons and liga- 

 ments are attached to bone, their fibres are prolonged through the 

 periosteum into the bone as perforating fibres. 



The firniiicss of the adherance of periosteum to ])one varies for 

 different bones and for dift"erent parts of the same bone. In young 

 growinj5 bone the j)resence of an inner osteogenetic layer results in a 

 rather loose attachment of periosteum. As bones grow older the 

 periosteum becomes more and more firmly attached. 



Bone Marrow 



Bone marrow is a soft tissue which occupies the medullary and 

 Haversian canals of the long bones and fills the spaces between the 

 trabecular of spongy bone. It consists of a delicate reticular connec- 

 tive tissue, in the meshes of which lie various kinds of cells. In large 

 marrow cavities there is a more or less definite arrangement of the 

 reticular fibres to form a thin lining membrane known as the endos- 

 teum. Because of its function as a blood-forming organ it contains 

 all varieties of blood cells as well as certain other cells. It is con- 

 venient to divide marrow cells primarily into (i) blood cells, adult 

 and developmental forms, and (2) other cells. 



f , , , f / Non-granular leucocytes 



White ^"^"-^^^ ^^^'^'^ \ Granular leucocytes' 

 Blood cells J l Developmental forms — myelocytes 



r Adult non-nucleated 

 ., „ Red S T^ 1 .If / Primary ervthroblasts 



Marrow cells \ [ | Developmental forms | Secondary erythroblasts 



f Giant cells ^ Megakaryocytes 

 Uiant cells ^ polykaryocytes 



Other cells \ Fat cells 



I Plasma cells 



[ Cells of the reticular tissue 



The following description omits the adult forms of blood cells for 

 which the student is referred to p. 107. 



(i) Myelocytes.— These resemble the mononuclear and some of the 

 transitional forms of leucocytes. The nucleus is large and may be 

 lobulated. It contains a comparatively small amount of chromatin 

 and therefore stains faintly. The cytoplasm is finely granular and 

 stains with neutrophile dyes. Myelocytes are not present in normal 



