THE SKELETON. 31 



Ephippus, there are no radiated cells ; the dense tissue is traversed 

 by parallel undulating i:)lasmatic tubes, which here and there present 

 slight dilatations, divide, and give off minuter tubes which anastomose 

 in their interspaces. The medullary canals, from which the tube 

 derive their plasma, are few and large. 



Almost endless are the minor modifications of the structure of the 

 osseous tissue of the Vertebrate animals, chiefly produced by varieties 

 in size, course, and number of the vascular canals and the radiated 

 cells and tubes : both vascular canals and radiated cells may be ab- 

 sent in the portion of bone examined ; but the plasmatic tubes are 

 always present. In the dermal bone-plates of the sturgeon, they 

 become, in the dense exterior layer, as minute as in the so-called 

 enamel of the shark's teeth.* 



The growth of bone presents some modifications in the different 

 classes of Vertebrate animals. 



In Fishes the bones continue to increase in dimension almost 

 throughout life : this is best seen in the cranium, where the periphery 

 of the bones, both of those which overlap by squamous sutures, and 

 those which interlock by broad dentated surfaces, is cartilaginous, 

 and, in the thin bones, sub-transparent. Here the development, 

 serial arrangement and metamorphoses of the cartilaginous cells, in 

 other words, the growth of temporary cartilage, are abvays to be 

 seen in progress. 



The long bones of most Reptiles retain a layer of ossifying cartilage 

 beneath the terminal articulating cartilage, and growth continues at 

 their extremities throughout life. Few of the long bones of Birds have 

 separate terminal pieces or epiphyses : the distal epiphysis of the tibia 

 is an exception to this rule ; but the distinct single piece which forms 

 the upper end of the ankle-bone in the young bird represents the tarsal 

 segment, and rests, not on a single diaphysis, but on the still separate 

 proximal ends of the three metatarsals. In tail-less Batrachians and 

 in most of the Mammalian class, the ends of the long cylindrical bones, 

 which support the articular cartilages, are distinct in the growing 

 bone from the shaft, and arc termed " epiphyses," the shaft being the 

 " diaphysis : " the seat of the active growth of the bone is in a carti- 

 laginous crust at the ends of the diaphysis. Wlien the epiphyses 

 finally coalesce with the diaphysis, growth in the direction of the 

 bone's axis is at an end : but in the Mammalian bones, as in those of 

 Birds and Reptiles, there is a slower growth going on over the entire 

 periphery of the bone, which is covered by the periosteum : the 



* Tlie work of Bibra (iv.) contains good observations and illustrations of the 

 comparative microscopic anatomy of the osseous tissue in the different classes of 

 Vertebrata. 



