22 GROWTH OF BONES. 



trically around the vascular canals, around the entire cir- 

 cumference of the long bones, and in interrupted plates, 

 connecting together the walls of the vascular canals, so 

 as often to give rise to a reticular disposition of the bony 

 substance. 



In fishes, the bones continue to grow throughout life; 

 and their periphery, whether in the flat bones of the head 

 which overlap each other, or the thicker bones that inter- 

 lock, is cartilaginous or membranous, and the seat of pro- 

 gressive ossification. The long bones of most reptiles 

 retain a layer of ossifying cartilage beneath the terminal 

 articular cartilage; and growth continues at their ex- 

 tremities while life endures. Some of the long bones in 

 frogs, birds, and most of those in mammals, have their 

 ends distinct from the body or shaft of the growing bone, 

 these separately ossified ends being termed " epiphyses." 

 The seat of the active growth of the shaft is in a cartila- 

 ginous crust at the ends supporting the epiphyses ; when 

 these coalesce with the shaft, growth in the direction of 

 the bones' axis comes to an end; but there is a slower 

 growth going on over the entire periphery of the bone, 

 which is covered by a membrane called the "periosteum." 

 In this membrane, the vascular S3^stem of a bone, except 

 the vessel supplying the marrow-cavity, undergoes the 

 amount of subdivision which reduces its capillaries to 

 dimensions suited for penetrating the pores leading to the 

 vascular canals. 



Thus bone is a living and a vascular part, growing by 

 internal molecular addition and change, and having the 

 power of repairing fracture or other injury. The shells 

 and crusts of molluscous and crustaceous animals are un- 

 vascular ; they grow by the addition of layers to their 

 circumference, may be cast off when too small for the 



