COMPARED WITH THOSE OF ANIMALS. 69 



order to distinguish them from, those earthy accumulatioas 

 celled sclerbgen, which produce the more indurated tissues 

 investing the seeds of plants. The strength of the stem 

 and root depends on the lignine accumulated in the fibrous 

 portion of their tissues ; and the fibrous tissues, thus solidi- 

 fied, coiiatitute the skeleton or frame-work of the plant. 



In the same manner, earthy matter accuniulafes more 

 rapidly in some of the tissues of the animal body than in 

 others ; in bone for instance, which thus becomes solid and 

 hard, and peculiarly fitted to support the softer part of the 

 animal frame work. Like the ligneous system of plants, the 

 osseous system of animals is the last to arrive .at maturity, 

 the process of ossification in the human skeleton not being 

 complete until about the 16th or 18th year. The time of 

 its commencement and completion, varies in different parts 

 of the skeleton. The bones first formed are those which 

 enclose the central organs of circulation and the nervous 

 system. The ossification of the extremities takes place at 

 a later period. 



Bone is composed of animal and earthy matter, the 

 former consists of gelatine, the latter of phosphate of lime. 

 If bone be burnt in a clear fire for about fifteen minutes, 

 the animal matter is destroyed, and the earthy matter 

 remains as a white and brittle substance, the bone retaining 

 its form ; on the other hand, if bone be digested in mu- 

 riatic acid for a few days, the earthy matter is entirely re- 

 moved, and the animal matter remains, as a tough elastic 

 substance, which can be bent in any direction. 



In childhood and youth the animal matter preponderates 

 over the earthy. The gelatinous and flexible bones of the 

 extremities of a child curve outwardly from this cause, as 

 they are too weak -to support the weight of its body, and 

 sometimes become permanently deformed if the child is 



