320 Basic Structure of Vertebrates 



tissues accompanied by vascular tissues. In the brain and spinal cord 

 of vertebrates occurs not only the usual mesenchymal connective tissue 

 but also another, which is unique in that its cells have ectodermal origin 

 in common with the nerve-cells. Some of the cells of this neuroglia 

 possess branched processes which make them appear confusingly 

 similar to nerve-cells. The neuroglia cells form, by means of their 

 processes, a supporting network for the nerve-cells. 



A nerve is a bundle of neuraxons, each of which may be ensheathed 

 as described above, and all wrapped together within a sheet of con- 

 nective tissue, the perineurium (Fig. 262), extensions of which (endo- 

 neurium) may penetrate into the bundle. Larger nerves consist of 

 several or many bundles all tied together by connective tissue and en- 

 wrapped by a relatively thick epineurium. Small blood-vessels trav- 

 erse the connective-tissue layers of the nerve. 



TISSUES SERVING FOR MECHANICAL SUPPORT 



Protoplasm is a substance of semifluid or gelatinous consistency. 

 An elephant constituted of protoplasm only is a mechanical impossi- 

 bility. Large animals, especially if they are land animals, require 

 mechanical support. Protoplasm provides such support by appropri- 

 ating various materials from the environment and building them into 

 nonliving structures which are external to the cells and physically 

 adapted to the mechanical needs of the animal as a whole and of its 

 parts. Just as man uses steel wires, various cements and masonries, 

 so protoplasm has its connective-tissue fibers, intercellular cements, 

 and that most delicate calcareous masonry, bone. 



The basis of the material of these supporting structures consists of 

 various nitrogenous or protein substances. By impregnation of the 

 material with inorganic salts, chiefly those of calcium, hard or rigid 

 supporting structures are produced. The protoplasmic or cellular 

 agencies concerned in building the supporting tissues are mesenchyme 

 cells, except in the cases of the notochord and the ectodermal neuroglia 

 of nervous organs. 



The embryonic precursor of supporting tissues other than the ex- 

 ceptions mentioned is a more or less spongy mesenchyme (Fig. 263/1 ) 

 whose individual cells have branching processes by means of which the 

 cells are joined together. The spaces within the meshwork of cells are 

 filled by a homogeneous fluid substance, the matrix. Presumably the 

 cells are the source of the matrix. 



Connective Tissue 



The essential mechanical structures in connective tissue are rela- 

 tively coarse white fibers consisting of an albuminoid substance, colla- 



