DEVELOPMENT OF BONE 175 



Blood Supply. Marrow, and especially the red variety, is richly 

 supplied with blood. The nutrient arteries penetrate to the mar- 

 row cavity of the bone and supply an abundance of small arteries 

 to all portions of the medulla. The arteries terminate in broad 

 capillary vessels whose wide lumen and delicate endothelial walls 

 determine their character as sinusoids. It was formerly thought 

 that the endothelial walls of these vessels were here and there defi- 

 cient, and, although recent investigations discredit the former 

 observations, the all-important fact remains, that the endothelial 

 walls are pervious to both red and white blood cells, neither is this 

 the only location where the red as well as the white cells may, 

 under certain conditions at least, penetrate the endothelial walls 

 of the blood capillaries. 



Efferent veins return the blood from the sinusoidal capillaries 

 of the marrow. These veins, as also those of the bony tissue, are 

 not supplied with valves. 



The lymphatics of bone occur in great abundance in the peri- 

 osteum, and as perivascular spaces penetrate the canals of Havers 

 and Volkman and thus reach the medullary cavity. The exist- 

 ence of lymphatics within the marrow, other than in the sheaths 

 of the blood vessels, is doubtful. 



The nerves accompany the blood vessels in all portions of the * 

 bone and marrow, and form a rich perivascular plexus which is 

 distributed to the walls of the vessels ; occasional side fibrils are 

 also distributed to the marrow. Nerve endings have not been 

 demonstrated in compact bone. In the periosteum terminal nerve 

 fibrils are supplied to the musculature of the blood vessels, and 

 other sensory fibrils end in Paccinian corpuscles. 



DEVELOPMENT OF BONE. Bone makes its appearance very 

 early in fetal life. The long bones are mapped out by masses of 

 fetal hyaline cartilage as early as the embryo begins to acquire its 

 typical form. The entire skeleton, with the exception of the flat 

 bones of the trunk and those of the vault of the skull and face, 

 are thus primarily formed by plates of fetal cartilage. The pro- 

 cess by which these cartilaginous plates are formed into bone is 

 known as intracartilaginous ossification. 



The flat bones, together with most of those of the face, are 

 formed directly from the mesoblastic connective tissue without 

 the intervention of cartilage. This method of bone formation 

 (litters somewhat from the above and is known as intramemlranous 

 ossification. 



