70 OSTEOLOGY. 
corpuscles of the blood of the embryo. It is these cells (erythroblasts) which are 
concerned in the formation of the coloured corpuscles of the blood. Marrow which 
displays these characteristic appearances is distinguished from the yellow variety 
already described by being called the red marrow. In the diploe of the cranial 
bones of aged individuals the marrow, which has undergone degenerative changes, 
is sometimes referred to as the gelatinous marrow. 
Apart from the adaptation of form rendered necessary by the use to which the 
bone is put, external influences are seen to react upon the intimate structure of 
the bone itself. Thus, if sections of different bones be made, the arrangement of 
their cancellous and dense tissue is seen to vary. In long bones the walls of 
the shaft are thick and strong, more particularly towards the concave side, if the 
shaft happens to be bent. The marrow cavity—largest towards the centre— 
gradually tapers towards the extremities, being encroached upon by the surround- 
ing cancellous tissue, which is disposed in lnes converging towards the extremities 
like the sides of a vaulted arch, thereby forming platforms on which the epiphyses 
are supported. The surfaces of these platforms are not smooth, but so arranged 
as best to withstand the strain to which the epiphyses are habitually subjected. 
Such provision is necessary in order to obviate the tendency to separation, which 
would otherwise occur prior to the complete osseous union of the diaphysis with 
the epiphyses. In the epiphysis itself the arrangement of the fibres of the 
cancellous tissue is determined by the disposition of the articular surfaces. The 
osseous lamelle, as a rule, are disposed at right angles to the planes of the articular 
facets, whilst they are bound together by other lamellze arranged conformably with 
these articular planes. The former correspond to the direction of greatest pressure, 
whilst the latter agree with the lines of greatest tension. In cases where there is 
an outstanding process projecting from the shaft, as, for example, the head and 
neck of the femur, a section of the bone displays a bracket-like arrangement of the 
osseous fibres of the cancellous tissue, which assists materially in strengthening the 
bone. 
Ossification and Growth of Bones.—For an account of the earlier development 
of the skeleton the reader is referred to pp. 28 and 45. Concerning the subsequent 
changes which take place, these are dependent on the conversion of the sclerato- 
genous tissue into membrane and cartilage. A characteristic of this tissue is that 
it contains elements which become formed into bone- producing cells, called osteo- 
blasts. These are met with in the connective tissue from which the membrane 
bones are formed, whilst they also appear in the deeper layers of the imvesting 
tissue of the cartilage (perichondrium), and so lead to its conversion into the bone- 
producing layer or periosteum. All true bone, therefore, may probably be regarded 
as of membranous origin, though its appearance is preceded i in some instances by 
the deposition of cartilage ; in this case calcification of the car tilage is an essential 
stage in the process of bone formation, but the ultimate conversion into true bone, 
with characteristic Haversian systems, leads to the absorption and disappearance of 
this primitive calcified cartilage. 
Membrane bones are such as have developed from fibrous tissue without 
having passed through a cartilaginous stage. Of this nature are the bones of the 
cranial vault and the majority of the bones of the face, viz. the superior maxillee, 
malars, nasals, lachrymals, and palate bones, as well as the vomer. The internal 
pterygoid plate i is also of membranous origin. 
Cartilage bones are those which are preformed in cartilage, and include most 
of the bones of the skeleton. Their growth is often described as endochondral 
and ectochondral, the former term implying the deposition of membrane bone in 
the centre of the cartilage, while the latter signifies a deposit of membrane bone on 
the surface of the cartilage, the osteogenetic layer on the surface of the cartilage 
being called the perichondrium till once bone has been formed, when it is called 
the periosteum. 
In the course of the development of a bone from membrane, as, for example, 
the parietal bone, the fibrous tissue corresponding to the position of the primary 
centre becomes osteogenetic, because here appear the bone-forming cells (osteoblasts), 
which rapidly surround themselves with a bony deposit more or less spicular in 
