26 MAMMALIAN ANATOMY 



projections at various places mark the firm attachments of muscles and 

 ligaments. All openings in the surface have been divided into three 

 grades : first, the microscopically small openings scattered everywhere 

 over the surface and giving entrance to vessels, nerves, and fibres from 

 the periosteum, the investing membrane of the bone ; second, larger 

 openings at the ends of bones for larger vessels and nerves ; and, third, 

 one still larger opening, situated in the shaft, usually nearer one end 

 than the other, and piercing the compact tissue obliquely. This is the 

 nutrient foramen, for the principal artery and its accompanying veins. 

 Internal Structure. If the bone be sawed lengthwise and the inte- 

 rior exposed, the central part, or shaft, will be seen to be a tube of 

 dense compact tissue surrounding the medullary cavity. Near the 

 ends of the bone this compact tissue thins out to a mere crust by 

 giving off internally plates and rods, which crossing and interlacing 

 form the cancellous tissue of the ends, with the meshes whereof the 

 medullary cavity of the shaft freely communicates (Fig. 9, a). If the 

 bone be a young growing bone, a transverse line of epiphyseal car- 

 tilage will be observed separating the shaft from the ends ; if the bone 

 be from an older animal, the original position of the cartilage will be 

 indicated bv a line of denser tissue. 



/ 



The interior of a flat bone, such as the scapula, is composed of two 

 layers of compact tissue separated by a layer of cancellous tissue, and 

 in very thin places of a single layer of compact tissue. In the flat 

 bones of the skull the cancellous tissue is known as the diploe, 1 and 

 the limiting layers of compact tissues are known as the external and 

 internal tables. 



One of the short bones should be now examined, and by a cross- 

 section the interior, composed of cancellous tissue limited by an exter- 

 nal crust of compact tissue, exposed (Fig. 9, b}. 



FRESH BONE. A bone in the fresh or natural state presents a dif- 

 ferent appearance from a dried bone, and shows parts which the latter 

 has lost in the processes of cleaning. 



If a fresh bone of a cat be not conveniently available, an arm-bone 

 of a sheep, or one of the smaller beef-bones, which can be always 

 obtained in market, will serve equally well. A second bone, split 

 lengthwise by the butcher, will be necessary for the examination of 

 the interior. 



1 From (Gr.) diploos, double. 



