THE STUDY OF ORGANS. 43 



which appear alternately light and dark under the 

 microscope, because the substance of the- rings pos- 

 sesses the peculiar optical property known as " double 

 refraction." These muscles are formed of fibres, in 

 which the original cellular structure is lost. The 

 striped muscles are best seen in insects, which seem 

 to have tbem brought to the greatest perfection both 

 in form and use, since insects are the strongest animals 

 known, for their size. 



We must not look upon muscles as mere fiUing-up 

 stuff ; they may be called the organs of action ; 

 they are the special tools of the will, and are all the 

 more interesting because of their curious and com- 

 plicated structure, which is as yet not thoroughly 

 understood. 



To the mesoblast layer belongs also the dermis, 

 or lower layer of the skin of vertebrates, with the 

 various structures derived from it. 



The muscles in vertebrate animals are supported by 

 bones. Bone consists of cartilage or gristle, in which 

 mineral matter has been deposited; this process is 

 accomplished by special cells, called osteoblasts. In 

 the body of the higher vertebrates, nearly all the 

 gristle is thus turned into bone in the adult animal; 

 but in some of the lower fishes the bulk of it remains 

 gristle. In the invertebrates, bones are not found, 

 except in the case of the cuttlefish. There are a few 

 bones which are not ossified from gristle, but from a 

 kind of fibrous tissue or membrane. They are scales 

 belonging to the lower skin or dermis, which have 



