ib-,4 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



vascular zones with little functional overlap, the ob- 

 struction of which on either arterial or venous sides 

 is a factor in osteochondritis juvenilis (19). 



The role of the periosteum in bone regeneration 

 and the incorporation of blood vessels from the 

 periosteal vascular network during bone growth in 

 width can still be accepted in that the vitality of the 

 osteogenic layer of young periosteum is maintained 

 by the osteogenic capillary layer fed by periosteal 

 arteries (54). 



As visualized radiographically, the venous system 

 differs somewhat from the arterial system. Thus, a 

 solitary longitudinal channel of wide caliber in an 

 approximately central medullary position can be 

 traced from one end of a bone to the other. Since this 

 central canal lacks a muscular tunica media (88), it 

 may properly be called a central venous sinus in the 

 medulla (16). At the trochanteric fossa the central 

 venous channel is joined by tributaries from the lesser, 

 third, and greater trochanter as well as by a vessel 

 p.issing down the neck from the head of the femur. 

 The central vein is joined by the principal nutrient 

 vein a short distance below the nutrient canal and 

 passes characteristically as a single vessel down to the 

 inferior metaphysis where it anastomoses with an 

 ascending branch of the middle geniculate vein. 

 Sometimes it divides into two stems at the mid-shaft 

 level. The central venous channel has numerous 

 transverse branches radiating toward the endosteum 

 and these drain the sinusoids of the medulla. An 

 endosteal line marking the junction of the medullary 

 sinusoids with the cortical capillaries can be seen. 



Branemark (11), who has been able to visualize 

 bone circulation in the living rabbit, states that the 

 bone marrow arteriole divides dichotomously into 

 capillaries. These run to sinusoids which are sometimes 

 hexagonal, sometimes spindle shaped. Sinusoids may 

 unite to form sinusoidal systems. The sinusoids are 

 drained by venules into collecting venules which 

 empty into the central veins. The sinusoids vary 

 rhythmically in their degree of dilation. Blood cells 

 may bypass a sinusoid by flowing through a shunting 

 capillary directly into a venule. In some instances, 

 cells appear to hug the vessel wall of one half of a 

 sinusoid apparently without disturbing flow in the 

 other half. Capillaries stemming from marrow ar- 

 terioles enter the Haversian canals to supply endos- 

 teal parts of diaphyseal bone. The capillaries then 

 swine; back into the marrow to empty into sinusoids 

 or directly into collecting venules. Blood flow in bone 

 capillaries is fairly steady, and the velocity of flow is 

 higher than in marrow capillaries. 



In the pigeon, "transitional capillaries" (37, 38) 

 connect the arteries to the venous sinusoids. The 

 capillary link is extremely circumscribed, and it is not 

 until the venous sinusoidal anastomoses are reached 

 that the blood spreads out in lacing and interlacing 

 vessel tufts, thence to be directed from the tuft-like 

 branchings into larger and larger vessels eventually to 

 enter the central longitudinal vein almost at right 

 angles. 



There seems little doubt that the extensively dis- 

 tributed, spacious, thin-walled venous sinusoids nor- 

 mally form the principal functioning vascular bed for 

 the actively circulating blood in marrow, i.e., they 

 correspond to the capillaries of other organs. 



In pigeons in which the marrow is made hypo- 

 plastic by starvation, one can see, between the fat 

 spaces, well-outlined and clearly defined channels 

 which constitute a most extensive system of capil- 

 laries (37). Many of these appear to be nonpatent and 

 functionally dormant as far as the active blood circu- 

 lation is concerned. These capillaries come off the 

 venous sinusoids by way of conical openings, and seem 

 to be continuous with them. They are not capillaries 

 in the sense of an arteriovenous transition, but instead 

 extend from venous channel to venous channel; they 

 are intersinusoidal. The same intersinusoidal semi- 

 collapsed channels have been reported in the marrow 

 of the ribs of the white rat 138), and are believed to be 

 present in the dog (40). 



Three theories as to the nature of the circulation in 

 adult marrow have been advanced : a) by Rindfleisch 

 (98) who believed that the blood spaces are lined by 

 parenchvma alone and have no endothelial cells; b) 

 Langer (81), on the other hand, thought of the mar- 



Spinal 

 branches 



Ddrsc^i 

 medial a. 



Aorta- 



Calcified 

 spongiosum 



Cartilaginous 



Anterolateral a. 



fig. 4. Diagram of a transverse section through the mid- 

 body of the vertebra of a six-month-old fetus. [From Ferguson 

 (46).] 



