g32 DEVELOPMENT AND GROWTH OF BONE. 



which is identical with and ultimately becomes the periosteum. The formation of bone, when 

 nreceded bv cartilage is called endochondral bone. Some bones, such as the tabular bones of 

 the vault of the cranium, the facial bones, and part of the lower jaw, are not preceded by 

 cartilage In the latter there is merely a membrane present, while from and in it the future bone 

 is formed. It becomes the future periosteum as well. This is called the intra-membranona 

 or periosteal mode of formation.] . 



[Endochondral Formation. (1) The cartilage has the shape of the future bone only m 

 miniature and it is covered with periosteum. In the cartilage an opaque spot or centre of 

 ossification appears, due to the deposition of lime-salts in its matrix. The cartilage cells 

 proliferate in this area, but the first bone is formed under the periosteum in the shaft, so 

 that an osseous case like a muff surrounds the cartilage. This bone is formed by the sub- 

 periosteal osteoblasts. (2) Blood-vessels, accompanied by osteoblasts and connective -tissue, 

 tow into the cartilage from the osteogenic layer of the periosteum {periosteal processes of 

 Virchow), so that the cartilage becomes channelled and vascular. As these channels extend they 

 open into' the already enlarged cartilage lacunae, absorption of the matrix taking place, while 

 other parts of the cartilaginous matrix become calcified. Thus a series of cavities, bounded by 

 .alcitied cartilage the primary medullary cavities are formed. They contain the primary or 

 cartilage marrow, consisting of blood-vessels, osteoblasts, and osteoclasts, earned in from the osteo- 

 genic layer of the periosteum, and of course the cartilage cells that have been liberated from their 

 lacuna. (3) The osteoblasts are now in the interior of the cartilage, where they dispose them- 

 selves on the calcified cartilage, and secrete or form around them an osseous matrix, thus enclosing 

 the calcified cartilage, while the osteoblasts themselves become embedded in the products of their 

 own activity and remain as bone -corpuscles. Bone therefore is at first spongy bone, and as the 

 primary medullary spaces gradually become filled up by new osseous matter it becomes denser, 

 while the calcified cartilage is gradually absorbed. It is to be remembered that, pari passu 

 with the deposition of the new bone, bone and cartilage are being absorbed by the osteoclasts.] 



Chemical Composition of Bone. Dry bone contains of organic matter or ossein, from which 

 gelatin can be extracted by prolonged boiling ; and about mineral matter, which consists of 

 neutral calcic phosphate, 57 per cent.; calcic carbonate, 7 per cent.; magnesic phosphate, 1 to 

 2 per cent. ; calcic fluoride, 1 per cent, with traces of chlorine ; and water, about 23 per cent 

 The marrow contains fluid fat, albumin, hypoxanthin, cholesterin, and extractives. The red 

 marrow contains more iron, corresponding to its larger proportion of haemoglobin (Nasse). 



[The medullary cavity of a long bone is occupied by yellow marrow, which contains about 

 96 per cent, of fat The red marrow occurs in the ends of long bones, in the flat bones of the 

 skull, and in some short bones. It contains very little fat, and is really lymphoid in its 

 characters, being, in fact, a blood-forming tissue (p. 12).] 



Growth of Bones. Long bones grow in thickness by the deposition of new bone from the 

 l>eriosteum, the osteoblasts becoming embedded in the osseous matrix to form the bone-corpuscles. 

 Some of the fibres of the connective-tissue, which are caught up, as it were, in the process, 

 remain as Sharpey's fibres, which are calcified fibres of white fibrous tissue, bolting together the 

 peripheric lamelhe. [Miiller and Schafer have shown that there are also fibres in the peripheric 

 lamellae, comparable to yellow elastic fibres ; they branch, stain deeply with magenta, and are 

 best developed in the bones of birds.] 



[At the same time that bone is being deposited on the surface, it is being absorbed in the 

 marrow cavity by the action of the osteoclasts, so that a metallic ring placed round a bone in a 

 young animal ultimately comes to lie in the medullary cavity (Duhamel). The growth in 

 length takes place by the continual growth and ossification of the epiphysial cartilage. The 

 cartilage is gradually absorbed from below, but it proliferates at the same time, so that what is 

 lost in one direction is more than made up in the other (J. Hunter). 



When the growth of bone is at an end, the epiphysis becomes united to the diaphysis, the 

 epiphysial cartilage itself becoming ossified. It is not definitely proved whether there is an 

 interstitial expansion or growth of the true osseous substance itself, as maintained by Wolff 

 (244,9). 



[Howship's Lacunae. The osteoclasts or myeloplaxes are large multinuclear giant-cells, 

 which erode bone. They can be seen in great numbers lying in small depressions corresponding 

 to them Howship's lacunae on the fang of a temporary tooth, when it is being absorbed. 

 They are readily seen in a microscopical section of spongy bones with the soft parts preserved.] 



The form of a bone is influenced by external conditions. The bones are stronger the greater 

 the activity of the muscles acting on them. If pressure acting normally upon a bone be 

 removed, the bone develops in the direction of least resistance, and becomes thicker in that 

 direction. Bone develops more slowly on the side of the greatest external pressure, and it is 

 curved by unilateral pressure (Lesshaft). 



448. DEVELOPMENT OF THE VASCULAR SYSTEM. Heart. [The heart appears as a 

 solid mass of cells in the splanchnopleure, at the front end of the embryo, immediately under 

 the u fore-gut." Very soon a cavity appears in this mass of cells ; some of the latter float free 

 in the fluid, while the cellular wall begins to pulsate rhythmically. This hollow cellular 



