GENERAL STRUCTURE OF ANIMALS. 



17 



scattered numerously through the bony matter are black 

 cavities. These are the lacuna. Running from these in 

 all directions, like the legs of an insect, are very slender 

 tubes connecting the lacunas with one another and with 

 the canals. These are the canalicles (canaliculi). The 

 red blood circulates freely through the canals, but the 

 blood globules are too large to pass the 

 canalicles. Therefore only colorless blood 

 plasma reaches the lacunas. 



As already said, bone is the material 

 of the skeleton of higher vertebrates, but 

 as we pass backward in the embryonic 

 series, or down in the animal series, car- 

 tilage replaces bone. In other words, 

 cartilage is the embryonic or imperfect 

 form of bone. 



Origin. — Bone comes usually from car- 

 tilage by deposit of mineral matter ; but 

 bone may also be formed by deposit of 

 mineral matter in other tissues, as in 

 fibrous membranes and in skin. So we 

 have cartilage bones, membrane bones, 

 and skin bones. 



Varieties. — Dentine is a denser kind of bone in which 

 the canals are wanting. This is the principal material 

 of teeth. Ivory is the finest example. Enamel is a still 

 denser variety of bone which covers the teeth of higher 

 vertebrates (Fig. 8). 



We must bear in mind the distinction between a 

 bone as an organ and bone as a tissue or material of 

 which the organ is composed. 



4. Muscle Tissue. — We must also distinguish here 

 between a muscle as an organ and muscle as a tissue. A 

 muscle is an organ consisting of several tissues — for ex- 

 ample, connective, nervous tissue, etc. — but its charac- 



Fig. 8. 



Section of tooth. 



