civ BONE. 



extended, gets thicker and closer in texture, especially towards the centre, 

 and the larger bony spicula which now appear, run out in radiating lines to 

 the circumference. The ossification continues thus to spread and consolidate 

 until the parietal meets the neighbouring bones, with which it is at length 

 united by suture. 



The figure (XLIX.) represents the parietal bone of an embryo sheep about 

 two inches and a half long, and shows the character of the ossification as 

 it appears when the object is magnified about twelve diameters. The bone 

 is formed in membrane as in the human foetus, but a thin plate of cartilage 

 rises up on its inside from the base of the skull. The ossification, however, 

 is decidedly unconnected with the cartilage, and goes on in a membrane 

 lying outside of it. The cartilaginous plate is not represented in the figure, 

 but a dotted curve-line, a, b, near the top, marks the height to which it 

 reached, and from this it will be seen that the ossification extended beyond 

 the cartilage. In the region of the frontal bone the cartilage does not even 

 rise so high. In both cases its limit is well defined, and under the micro- 

 scope it presents a decided contrast to the adjacent membrane. 



When further examined with a higher magnifying power, the tissue or 

 membiane in which the ossification is proceeding, appears to be made up 

 of fibres and granular corpuscles, wich a soft amorphous or faintly granular 

 uniting matter, and, in point of structure, might not unaptly be compared 

 to connective tissue in an early stage of development. The corpuscles are 

 large, mostly two or three times the size of blood-corpuscles ; their sub- 

 stance is granular in character, and, especially in specimens preserved in 

 spirit, usually hides the nucleus. They are densely packed all over the 

 area of ossification, covering the bony spicula, and filling up their inter- 

 stices ; so that, to bring the growing parts into view, the corpuscles must 

 be washed away with a hair pencil, or removed by short immersion of the 

 specimen in weak solution of soda. 



On observing more closely the points of the growing osseous rays at the 

 circumference of the bone, where they shoot out into the soft tissue, it will 

 be seen that the portion of them already calcified is granular and rather dark 

 in appearance (fig. L., a, b, c), but that this character is gradually lost as 

 they are traced further outwards in the membrane, in which they are pro- 

 longed for a little way in form of soft and pliant bundles of transparent fibres 

 (fig. L.,/). Further inwards, where the slender rods or bars of bone are 

 already in great part hard, their calcified substance is coated over (although 

 unequally) with transparent and as yet soft and imperfectly calcified matter, 

 by which they grow in thickness ; and this ossifying substance spreads out 

 at their sides, and encroaches on the intervening space, in form of a bright 

 trellis-work (fig. L., d), thin towards its outer limit, and there composed of fine 

 fasciculi, but denser and coarser nearer the bone, where the trabeculse are 

 thick and round, and already granular from commencing earthy impregna- 

 tion.* The interstices of this mesh- work are in some parts occupied by one 

 or more of the corpuscles, but at other parts they are reduced to short narrow 

 clefts or mere pores. The appearance which I have endeavoured to describe 

 is especially well seen at those places where a cross bridge of bone is being 

 formed between two long spicula (as at e) ; we may there distinguish the clear 



* A notion of this appearance may also be obtained from Fig, LVIII,, page cxiii., which 

 represents intramembranous ossification advancing under the periosteum of a long bone. 

 From a to c is the ossifying trellis-work, but coarser than in the early cranial bone. 

 From, a to b is the part already impregnated with earthy deposit, which is encroaching on 

 the part b, c, as yet soft and pellucid. 



