BONE. 385 



with each canal are an infinite number of wavy tubes, which are con- 

 nected with the canals in the same manner as the dentinal tubes of the 

 teeth are connected with the pulp-cavity ; and if such a specimen were 

 placed by the side of a section of the tooth of some of the Shark tribe, 

 the discrimination of one from the other would be no easy matter. In 

 the spine of a Ray the analogy between bone and the ivory of the teeth 

 is made more evident ; for in this fish we have tubes, like those of 

 ivory, anastomosing with the canaliculi of bone-cells. 



Having said thus much on the minute structure of the bone com- 

 posing the skeleton in the four vertebrated classes, let us proceed at 

 once to the application of the facts which have been laid down ; and 

 let us, for example, suppose that a fragment of bone of an extinct ani- 

 mal be the subject of investigation. It has been stated, that the bone- 

 cells in mammalia are tolerably uniform in size ; and if we take 

 l-2000th of an inch as a standard, the bone-cells of birds will fall below 

 that standard : but the bone-cells of reptiles are very much larger than 

 either of the two preceding ; and those of fishes are so entirely different 

 from all three, both in size and shape, that they are not for a moment 

 to be mistaken for one or the other ; so that the determination of 

 a minute yet characteristic fragment of fishes' bone is a task easily 

 performed. If the portion of bone should not exhibit bone-cells, but 

 present either one or other of the characters mentioned in a preceding 

 paragraph, the task of discrimination will be as easy as when the bone- 

 cells exist. We have now the mammal, the bird, and the reptile to 

 deal with ; in consequence of the very great size of the cells and their 

 canaliculi in the reptile, a portion of bone of one of these animals can 

 readily be distinguished from that of a bird or a mammal ; the only 

 difficulty lies between these two last : but notwithstanding that on a 

 cursory glance the bone of a bird appears very like that of a mammal, 

 there are certain points in their minute structure in which they differ ; 

 and one of these points is in the difference in size of their bone-cells. 

 To determine accurately, therefore, between the two, we must, if the 

 section be a transverse one, also note the comparative sizes of the 

 Haversian canals, and the tortuosity of their course ; for the diameter 

 of the canal bears a certain proportion to the size of the bone-cells, and 

 after some little practice the eye will readily detect the difference. The 

 fragments necessary for the purpose of examination are to be selected 

 with some little care ; and on the whole, a small chip (or two) from 

 the exterior of the shaft of a long bone is sufficient : but as many 

 fossil bones are coated with a layer of earthy deposit on their external 

 surface, it will be requisite to get beneath this deposit, as it very sel- 



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