10-2 



LOCOMOTION. 



[CHAP. v. 



Fig. 18. 



of carbonic acid. Filter the acid liquid after diluting it with water, 

 and add solution of caustic ammonia as long as the precipitate at first 

 formed continues to be redissolved by agitation ; then add solution 

 of acetate of lead till it no longer occasions any precipitate. The 

 dense white precipitate thus produced consists of phosphate of lead, 

 which melts before the blow-pipe, and on cooling assumes its cha- 

 racteristic crystalline structure. 



Through the solution, filtered from the phosphate of lead, pass 

 a stream of sulphuretted hydrogen to remove the excess of lead ; 

 warm the liquid, to drive off the superfluous gas, and filter : then 

 neutralize by ammonia, and add oxalate of ammonia as long as any 

 precipitate occurs ; abundance of oxalate of lime will fall as a white 

 powder. 



Evaporate the filtered liquid to dryness ; ignite the residue, and 



wash with hot water; the magnesia will be left behind in a pure form. 



In examining a section of almost any bone, we observe two 



varieties of osseous substance : the one dense, firm, compact, always 



situated on the exterior of the bone, either as a thin layer, or as 



a dense thick structure pos- 

 sessed of great strength ; the 

 other loose, reticular, spongy, 

 enclosing spaces or cells, 

 which communicate freely 

 Avith each other, an*d which, 

 being called cancelli, give to 

 this kind of osseous tissue the 

 name cancellated. These cells 

 are formed by an interlace- 

 ment of numerous bony fibres 

 and laminae, which, although 

 to a superficial observation 

 exhibiting an indefinite ar- 

 rangement, have neverthe- 

 less, in those bones which 

 have to support weight, a 

 more or less perpendicular 



Vertical section of the upper end of the Femur, shewing direction. The Cancellated 



structure of bone is always 



situated in its interior, enclosed and protected by the compact tissue. 



The relative situation of these varieties may be well seen in a 



vertical section of one of the long bones (fig. 18) . At the extremities, 



the cancellated texture is accumulated, invested by a thin lamella of 



