18 THE BONES. 



insertions of tendons and ligaments. Its thickness and adherence are not the 

 same everywhere ; it adheres most closely near the ends of bones. By its inner 

 face it corresponds to the surface of the bone, into which it sends prolongations 

 {arciform fibres) which ultimately become Sharpey's fibres ; by its external face, 

 it is continuous with the surrounding connective tissue and that of the muscular 

 aponeuroses. 



The periosteum may be resolved into two layers, though these are not always 

 very distinct. The superficial layer is essentially fibrous, and is formed by a 

 network of elastic fibres containing bundles of longitudinal fibres and cells. 

 The deep layer is a closer elastic network, with finer connective tissue fibres, and 

 a larger quantity of cells and vessels. This is called the osteogenous layer , 

 because of its functions. 



C. Medulla. — The medulla, or marrow, is a pulpy, fatty substance, which fills 

 the medullary canal and the areola of the spongy tissue of the bones, and 

 partly the Haversian canals. Somewhat consistent, and of a rosy tint in the 

 bones of young animals, the marrow becomes diffluent and yellow in the bones 

 of those advanced in age, except in the vertebrae of the Horse, Ox, Dog, etc., 

 and in the limbs of the Rabbit. In the first state, it is also mucous or fibrous in 

 the cranial and facial bones undergoing development, and, rosy in colour, it 

 only contains traces of fat ; while in the second, it has 96 per cent, of this 

 substance. The medulla of bones is composed of : 1st, some trabecul^e of 

 delicate connective tissue and a network of stellate cells, to support the vessels 

 and nerves ; 2nd, fat cells ; 3rd, particular cells, named by M. Robin medullo- 

 cells and myeloplaxes. 



The medullo-cells, abundant in the red or foetal marrow, are small cells with a 

 spherical or budding nucleus {Bizzozero's cells), analogous to the lymph cells ; 

 some are impregnated with hgemoglobine. With regard to the myeloplaxes, these 

 are enormous flat cells, irregular in outline, and containing a large number of 

 nuclei. Rare in the yellow marrow, they are more particularly found adhering 

 to the walls of the medullary canal, or the alveoli of the spongy tissue. 



D. Blood-vessels. — The arteries of bones belong to three orders — a distinction 

 based on their volume and the extent of their distribution. 



The arteries of the first order penetrate to the interior of the medullary 

 canal of long bones, by a particular orifice — the nutrient foramen. They soon 

 divide into two branches, which break up into a network that lines the walls 

 of the canal and enters the tissue of the medulla. This network communicates 

 with the arteries of the second order, which go to the spongy tissue of the 

 extremities of the long bones, penetrating them by the numerous nutritive 

 foramina that surround the epiphyses. Lastly, the arteries of the third order 

 are branches of the periostic network that enters the superficial Haversian 

 canals. These canals may be considered, strictly speaking, as a third category 

 of nutrient conduits. In the flat and short bones there are no arteries of the 

 first order. 



Veins accompany the arteries, and are always more voluminous than these ; 

 they frequently make their exit by special and very large openings at those 

 points where the spongy tissue is abundant. The veins of bones sometimes 

 exhibit saccular dilatations on their course. Certain veins in the cranial bones 

 have their walls partly or entirely channeled in the bony tissue ; they are lined 

 by a simple endothelium. 



E. Lymphatic vessels. — The existence of these cannot be affirmed. 



