288 - Multicellular Animals, Especially Man 



tains the nutrition of its own outlying fibers. 

 Thus an axon or dendron that is cut off 

 from continuity with its centron undergoes 

 degeneration, although a replacement of the 

 fiber may be effected by a new outgrowth 

 from the cut end. The centrons of the nerv- 

 ous system are aggregated in small masses, 

 called ganglia, and — to a much greater ex- 

 tent — in the brain and spinal cord. Nerves — 

 which lead to and away from the ganglia, 

 brain, and spinal cord — are made up of 

 axons, dendrons, or both, running together 

 in a common bundle. In many nerves, each 

 fiber is insulated from its neighbors by an in- 

 dividual sheath (Fig. 15-15) of fatty material, 



CENTRON < 



AXONK 



NUCLEUS 

 CYTOPLASM 



BRANCH 



NODE OF 

 RANVIER 



TERMINAL 

 BRANCHES 



which is called the myelin or medullary 



sheath. However, some nerves, particularly 

 in the autonomic system (p. 465), are non- 

 myelinated. 



Connective (Sustentive) Tissues. The con- 

 nective tissues bind the other tissues together, 

 giving substantial form to each organ, and 

 connecting and supporting the various or- 

 gans. In all connective tissues, the intercellu- 

 lar matrix is far more conspicuous than the 

 cells by which this matrix is secreted. In fact, 

 the character of the matrix mainly deter- 

 mines the propei ties of the particular kind 

 of connective tissue in question — as may be 

 noted in the following description. 



1 . Fibrillar Connective Tissues. The matrix 

 of the fibrillar tissues is characterized by the 

 presence of numerous nonliving fibers, which 

 form dense interlacing networks or thick 

 parallel strands. The fibroblasts, or cells 

 that form the matrix, are usually difficult 

 to see among the fibers. They are inconspicu- 

 ous amoeboid cells, capable of migrating to 

 the various parts of the tissue. The chemical 

 nature of the fibers differs in different con- 

 nective tissues. In general, two main types 

 are recognized: (a) white fibers, which are 

 relatively delicate and inelastic, and (b) yel- 

 low fibers, which are stouter and more elastic. 

 White fibrous connective tissue is widely dis- 



Fig. 15-15. A neuron, or nerve cell. 



Fig. 15-16. Hyaline cartilage. One to several carti- 

 lage-producing cells (chondroblasts) occupy each of the 

 spaces (lacunae) in the chondrin matrix. (From The 

 Living Body, by Best and Taylor. Holt, Rinehart and 

 Winston, Inc.) 



