44 



ANIMAL PHYSIOLOGY. 



which., in long bones, are found near the articular extremities. 

 The marrow cavities in the shafts of long bones may be 

 looked on as of the same description as the cancellations in 

 the spongy tissue at the extremities of the same bones, with 

 which they communicate. The marrow is vascular connec- 

 tive tissue of a delicate descrip- 

 tion, loaded with adipose cells, and 

 has usually a special artery, the 

 BO -called nutrient artery, which 

 pierces the bone, and supplies both 

 the marrow and the innermost part 

 of the osseous tissue. 



23. Bone is formed, as has been 

 ctated, either from cartilage or 

 fibrous tissue. All bones of con- 

 siderable thickness are originally 

 cartilaginous, and their ossification 

 begins in the centre of the mass. 

 The first step preliminary to this 

 process of ossification is the multi- 

 plication of vessels within cana]s, 

 and the absorption before them of 

 a certain amount of cartilaginous 

 matrix. When a section is made 

 Fig. 24. Vertical Section through the plane of contact of a 

 through the plane of ossifi- centre of ossification and the sur- 

 cation at the upper end of rounding cartilage, the cartilage- 

 S^^ht^tolS -rpuscies are seen arranged in 

 in vertical columns, and rows placed ^ vertically to the 

 altered matrix between ; b, plane of ossification j and be- 

 granular deposit spreading tween these rows there pro- 

 inspicules;c, true bone. ^ ^to the ^ matrix opaque 

 spicules, which consist of granules of calcified matter, dis- 

 tinct one from another, and reflecting the light. By a 

 further deposition of granules, the cartilage-corp'uscles become 

 hid from view and closely surrounded; and in some instances 

 mineral deposit takes place also within the capsules. By 

 still further deposition of mineral matter, the matrix becomes 

 homogeneous and transparent, and within the ossifying border 

 spaces are formed by absorption. Within these spaces there 



