THE STUDY OF THE BONES. 15 



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

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

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

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

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

 called the osteogenoics layer. 



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

 the medullary canal and the areolte of the spongy tissue of the bones. 

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

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

 In the first instance, 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 trabeculsB of delicate connective tissue, which serves to support 

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

 3rd, Particular cells, named by M. Eobin meduUo-cells and myeloplaxes. 



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

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

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

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

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

 tissue. 



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

 founded 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 nutritious foramen. They 

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

 walls of the caual and enters the tissue of the medulla. This network 

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

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

 the numerous nutritious foramina that surround the epiphyses. Lastly, the 

 arteries of the third order are branches of the periostic network which enter 

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

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

 category of nutritious 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 parietes entirely composed of osseous tissue. 



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

 cannot be afSrmed. 



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

 nerves ; the latter are always vaso-motory nerves. 



Almost constantly a somewhat voluminous nerve enters the medullary 

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

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

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

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

 as the vertebree, are especially remarkable for the numerous nerves they 

 receive. 



