THE MICROSCOPE IN ZOOLOGY. 89 



packed together ; the spines or scales in the others are 

 placed in regular whorls, the highest of which is .com- 

 posed of knobby spines, the whole shaft being " sur- 

 mounted by a curious circle of six or seven large fila- 

 ments attached by their pointed ends to its shaft, 

 whilst at their free extremities they dilate into knobs." 

 I have no doubt that you will find much to occupy 

 your attention, and to afford you delight in the exami- 

 nation of thin sections of bone. " Bone consists of a 

 hard and soft part ; the hard is composed of carbonate, 

 phosphate, and fluate of 

 lime, and of carbonate and 

 phosphate of magnesia, de- 

 posited in a cartilaginous 

 or other matrix ; whilst the 

 soft consists of that matrix, 

 and of the periosteum which 

 invests the outer surface of 

 the bone, and of the me- 

 dullary membrane which 



lines its interior or medullary cavity, and is continued 

 into the minutest pores." You can, if you like, pre- 

 pare the sections of bone for examination, or you can 

 buy specimens already ground and mounted at a small 

 cost. With a thin sharp saw you must make as thin 

 a section as possible, and this must be ground down 

 on a hone, or be rubbed between two smooth hones 

 till you get the desired tenuity. Let the specimen 

 be further polished on a piece of plate glass, in order 

 to obliterate the scratchings caused by the friction on 

 the hone. If it is a long bone you wish to examine, 

 you should make a longitudinal section ; if a flat bone, 

 the section should be made parallel to its surface. 

 You will then see it is traversed by a great number of 

 canals, called Haver sian canals, after their discoverer, 

 Havers. These canals are in connection with the 



