20 



THE CAT. 



[CHAP. ii. 



adjacent lacunae unite, and thus fluid can traverse every part of 

 the bone. 



The Haversian canals grow larger as they proceed inwards (in 

 such a hone as that of the thigh,) and open into still larger channels 

 and yet wider interspaces which are called cancelli, ultimately 

 merging into a hollow central part called the medullary cavity of the 

 hone because it contains that delicate fibrous tissue and fat which 

 constitutes marrow, as already mentioned. 



Some bones have their entire substance replete with cavities or 

 cancelli, and such are called cancellated or spongy. 



6. OSSIFICATION may take place either through pre-existing carti- 

 lage or through membrane, and in either case 

 blood-vessels advance into the pre-existing ma- 

 Ae terial, and therewith that material is absorbed 

 and disappears around them and is replaced by 

 calcareous substance. The lacunae are inter- 

 spaces which have been left uncalcified owing 

 to the presence there of certain cells. These 

 cells have sent out radiating processes (like 

 some of the connective-tissue cells, as already 

 noticed,) which have also escaped the general 

 calcification of the intercellular substance, and 

 thus the canaliculi have been produced. Thus 

 contents of the lacunae are truly bone- cells 

 or corpuscles. Bone tissue therefore is, except 

 as to its calcareous nature, very like connec- 

 tive tissue and cartilage. The bony substance 

 answers to the matrix of these other tissues, and 

 the "bone cells" to their corpuscles. When 

 the earthy matter of bone is dissolved their 

 original cellular contents may often be detected. 

 When a bone ossifies from cartilage, as all 

 thick bones do, the deposit begins in the form 

 of opaque granules of calcareous matter, which 

 Kg. G.-VERTICAL SECTION OF surround and sometimes invade the cartilage 

 CAT'S THIGH-BONE (FEMUR), capsules and form a dense and irregular osseous 

 e tissue, without lacunaa or canaliculi. Spaces are 

 ^^ en f OTme ^^ this substance by absorption, 

 and if these spaces largely accumulate, can- 

 cellated tissue is formed. The spaces may, 

 however, become filled with a fresh and secondary deposit of bone 

 in concentric rings round the blood-vessels, thus forming the 

 " compact bone " already described. 



When bone is formed from membrane, it assumes the compact 

 form, with lacuna? and canaliculi, at once, and is not preceded by 

 granular deposit. 



7. The GROWTH OF BONE takes place in various ways by the ossi- 

 fication of the inner layer of the periosteum surrounding it. In long 

 bones, which are preceded by cartilage, the ends remain for some 



