STRUCTURE OF BONES 21 



The spongy substance (Substantia spongiosa) consists of delicate bony plates 

 and spicules which run in various directions and intercross. These plates are 

 definitely arranged with regard to mechanical requirements, so that systems of 

 pressure and tension plates can be recognized, in conformity with the lines of pres- 

 sure and the pull of tendons and ligaments respectively. The intervals (marrow 

 spaces) between the plates are occupied by marrow. The spongy substance forms 

 the bulk of short bones and of the extremities of long bones; in the latter it is not 

 confined to the ends, but extends a variable distance along the shaft also. Some 

 bones (Ossa pneumatica) contain air-spaces or sinuses within the compact sub- 

 stance instead of spongy bone and marrow. In certain situations the two compact 

 layers of flat bones are not separated by spongy bone, but fuse with each other; 

 in some cases of this kind the bone is so thin as to be translucent, or may even 

 undergo absorption, producing an actual deficiency. 



The flat bones of the cranial vault and sides are composed of an outer layer of 

 ordinary compact substance, an inner layer of very dense bone, the tabula vitrea, 

 and between these a variable amount of spongy bone, here termed diploe. 



The periosteum is the membrane which invests the outer surface of bone, 

 except where it is covered with cartilage. It consists of an outer protective fibrous 

 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 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 ossium 

 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 acUpose tissue. 



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. 



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 

 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 medullary or nutrient artery enters at the so-called nutrient foramen 

 (Foramen 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 form perivascular channels in the periosteum and the 

 Haversian canals of the compact substance. Lymph-spaces exist at the periphery 

 of the marrow. 



The nerves appear to be distributed chiefly to the blood-vessels. Special 

 nerve-endings (Vater-Pacini corpuscles) in the periosteum are to be regarded as 

 sensory, and probably are concerned in mediating the muscle sense (Kopsch), 



