22 OSTEOLOGY 
layer, and an inner cellular osteogenic layer. During active growth the osteogenic 
layer is well developed, but later it becomes much reduced. The fibrous layer 
varies much in thickness, being in general thickest in exposed situations. The 
adhesion of the periosteum to the bone also differs greatly in various places; it 
is usually very thin and easily detached where it is thickly covered with muscular 
tissue which has little or no attachment. The degree of vascularity conforms to 
the activity of the periosteum. 
The endosteum is a thin fibrous membrane which lines the medullary cavity 
and the larger Haversian canals. 
The marrow (Medulla ossium) occupies the interstices of the spongy bone and 
the medullary cavity of the long bones. There are two varieties in the adult— 
red and yellow. In the young subject there is only red marrow (Medulla ossitum 
rubra), but later this is replaced in the medullary cavity by yellow marrow (Medulla 
ossium flava). The red marrow contains several types of characteristic cells and 
is a blood-forming substance, while the yellow is practically ordinary adipose tissue.! 
Vessels and Nerves.—It is customary to recognize two sets of arteries—the 
periosteal and the medullary. The former ramify in the periosteum and give off 
Fic, 2.—Cross-SECTION OF PROXIMAL THIRD OF SHAFT Fic. 3.—Cross-secTion or Distat THirp oF SHAFT OF 
or Ricut Humerus or Horse. Lerr Humerus or Horse. 
Section passes through nutrient foramen and canal. 
innumerable small branches which enter minute openings (Volkmann’s canals) on 
the surface and reach the Haversian canals of the compact substance. Other 
branches enter the extremities of the long bones and supply the spongy bone and 
marrow in them. In the case of the larger bones—and especially the long bones 
the large nutrient or medullary artery (Arteria nutricia) enters at the so-called nu- 
trient foramen (Ioramen nutricium), passes in a canal (Canalis nutricius) through 
the compact substance, and ramifies in the marrow; its branches anastomose with 
the central branches of the periosteal set. The larger veins of the spongy bone do 
not, as a rule, accompany the arteries, but emerge chiefly near the articular surfaces. 
Within the bone they are destitute of valves. The lymph-vessels exist as peri- 
vascular channels in the periosteum and the Haversian canals of the compact sub- 
stance. They also form a fine subperiosteal network, from which the larger vessels 
proceed, usually in company with veins. Lymph-spaces exist at the periphery of 
the marrow. 
The nerves appear to be distributed chiefly to the blood-vessels. Special nerve- 
1 Since yellow marrow is formed by regressive changes in red marrow, including fatty infiltra- 
tion and degeneration of the characteristic cells, we find transitional forms or stages in the process. 
In aged or badly nourished subjects the marrow may undergo gelatinous degeneration, resulting 
in the formation of gelatinous marrow. 
