STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 59 



common hyaline cartilage, is sometimes modified so as to resemble 

 fibrous tissue, sometimes the fibrillar and sometimes the elastic 

 form being produced. (Figs. 31 and 32.) 



Bone is probably the most advanced differentiation of the con- 

 nective-tissue group. The intercellular substance is characterized 

 by containing a great quantity of earthy or inorganic matter, 

 which gives the tissue its enormous strength. It is, moreover, 

 everywhere traversed by the processes of the cells lying in little 

 canals (canaliculi), which connect the spaces (lacunae) in which 

 the protoplasmic bone cells sojourn. 



In the formation of bone from fibrous or cartilaginous tissue 

 the original intercellular substance disappears, and a set of cells 

 with new formative powers come upon the field (Fig. 34). These 

 new cells (osteoblasts) cover the growing surface of the bone 

 and secrete and lay down in layers a new kind of intercellular 

 substance, which is the bone matrix. Here and there at wonder- 

 fully regular intervals an osteoblast ceases to secrete the calca- 

 reous intercellular substance, while its neighbors continue forma- 

 tive activity. Consequently, this osteoblast, or, as it may now 

 be called, young bone cell, becomes surrounded by calcareous 

 intercellular substance, and is thus permanently lodged in the 

 bone tissue. 



The Vascular System is developed in the middle germinal 

 layer with the earliest stages of the connective tissue. The blood 

 vessels, which are chiefly made up of connective tissues, soon 

 traverse all the parts of the body, and distribute the nutrient 

 fluid or blood. And even the blood may be considered as an 

 outcome of the connective tissues, since the cells of the blood are 

 at first formed from the mesoblast, and later from the connective- 

 tissue corpuscles. 



An arrangement of special cells, such as epithelial or muscle 

 cells, with a special function, constitutes an organ. However, in 

 the higher animals and man, an organ is almost invariably a 

 complex structure, having various tissues entering into its con- 

 struction. Thus a skeletal muscle is made up of a quantity of 

 muscle fibres held together by sheets of connective tissue, and 

 attached to bones by connecting bands. It is further traversed 



