THE STUDY OF THE BONES. 15 



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

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

 i mixture of connective and elastic fibres and plasmatic cells. The deep 

 ayer also contains a loose connective tissue, but more especially elastic 

 ibres, and more or less voluminous spherical or fusiform cells. This is 

 ailed the osteogenous layer. 



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

 he medullary canal and the areolae of the spongy tissue of the bones. 

 Somewhat consistent, and of a rose tint in the bones of the young animal, the 

 narrow becomes diffluent and yellow in the bones of those advanced in age. 

 n the first instance, it only contains traces of fat ; while in the second it has 

 6 per cent, of this substance. The medulla of bones is composed of: 

 st, Some trabeculae of delicate connective tissue, which serves to support 

 tie vessels and nerves ; 2nd, Fat either free or inclosed in vesicles ; 

 rd, Particular cells, named by M. Eobin medulla-cells and myeloplaxes. 



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

 rith a spherical nucleus; while the myeloplaxes are large, flattened, or 

 olyhedral elements of an irregular outline, containing a great number of 

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

 dhering to the walls of the medullary canal, or the alveoli of the spongy 

 issue. 



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

 ounded on their volume and the extent of their distribution. 



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

 anal of long bones by a particular orifice, the nutritious foramen. They 

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

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

 ommunicates with the arteries of the second order, which are destined to 



spongy tissue of the extremities of the long bones, penetrating them by 



numerous nutritious foramina that surround the epiphyses. Lastly, the 

 .rteries of the third order are branches of the periostic network which enter 

 le superficial Haversian canals. These canals which open in this manner 

 n the surface of the bones may be considered, strictly speaking, as a third 

 ategory of nutritious conduits. In the flat and short bones there are no 

 .rteries of the first order. 



Veins accompany the arteries, and are always more voluminous than 

 hese ; they frequently make their exit by special and very large openings 

 .t those points where the spongy tissue is abundant. The veins of 

 >ones sometimes exhibit saccular dilatations on their course. Certain veins 

 a the cranial bones have their parietes entirely composed of osseous tissue. 



Lymphatic vessels. The existence of these in the interior of bones 

 annot be affirmed. 



Nerves. These belong to the cerebro-spinal and ganglionic system of 

 lerves ; the latter are always vaso-motory nerves. 



Almost constantly a somewhat voluminous nerve enters the medullary 

 anal by passing through the nutritious foramen, and is distributed to the 

 aedulla. The compact tissue receives few nervous filaments; while, on 

 be contrary, the spongy tissue at the extremities of the long bones, as well 

 .s the short bones, obtains an abundant supply. Certain short bones, such 

 .s the vertebrsB, are especially remarkable for the numerous nerves they 

 eceive. 



