io 4 COMPARATIVE ANATOMY 



the calcareous granules. Calcified cartilage is found in 

 abundance in the skeletons of sharks and rays, and it must not 

 be mistaken for bone, which is not present in these fishes ; nor 

 must any confusion be made between calcified cartilage and 

 " cartilage bone," such as has been described as occurring in 

 the skull of the frog. A cartilage bone is one which has taken 

 the place of pre-existing cartilage, but has not been formed out 

 of it. The replacement of cartilage by bone will be described 

 after a consideration of the structure of bone itself. 



Bone consists of an inorganic or earthy and an organic or 

 animal part, intimately blended together. If a bone is steeped 

 in dilute acid the earthy salts are dissolved out, and a model 

 of the bone is left in a flexible, tough substance which is 

 converted into gelatin on boiling. This last property is 

 sufficient to distinguish the animal part of bone from 

 cartilage, for the latter, after prolonged boiling, yields a 

 substance of different chemical character called chondrin. 

 The earthy parts of bone may be separated by burning out 

 the animal part in a fire. They consist chiefly of phosphate 

 of lime, with a small proportion of carbonate and a trace of 

 fluoride of lime. Bone may be compact or spongy ; in the 

 latter case it is called cancellous. Cancellous bone, however, 

 is made up of fine bars and partitions which have the same 

 structure as compact bone. 



*3If a thin slice of bone is viewed under the microscope it no 

 longer appears dense and homogeneous, but is seen to be 

 perforated by a number of holes, and a longitudinal section 

 shows that these are the sections of a number of short 

 longitudinal canals called Haversian canals. The calcareous 

 substance is arranged round each Haversian canal in a series 

 of concentric lamellae. The concentric systems of adjacent 

 Haversian canals nearly or quite touch one another, but are 

 generally separated by vertical layers, which run parallel to 

 the long axis of the bone and extend to its surface, where 

 they form a peripheral layer. The arrangement of the 

 lamellae is made conspicuous by the presence of numerous 

 little dark cavities, which are disposed conformably to the 

 lamellae in which they occur. These dark spaces are called 

 the lacunae. Each is connected centrally with the Haversian 

 canal, peripherally with other lacunae by numerous fine 

 branching passages called the canaliculi. The Haversian 



