METABOLISM IN VERTEBRATES 



lengths and have branches which come off at varying angles; these are called 

 dendrites. Others are seen to be of uniform, small diameter, with branches at 

 right angles to the main fiber, and are surrounded by a myelin sheath contain- 

 ing compound lipids (Fig. 3.\8E); these are the axons. Nerve fibers branch 

 repeatedly at their ends, forming the terminal filaments. The terminal fila- 

 ments of axons, which often end in plate-like or club-like expansions, lie 

 against the dendrites and cell bodies of other neurons or on muscle and gland 

 cells (see Fig. 4.22, p. 114). Where there are only two cytoplasmic exten- 

 sions, or nerve fibers, the cell is called a bipolar neuron (Fig. 3.18/1 and B). 

 Cells with more than two fibers are known as multipolar neurons (Fig. 3.18C). 

 Such cells never have more than one axon. The cell bodies of neurons are 

 sometimes found in groups, or ganglia, outside the central nervous system; 

 other nerve cell bodies occur in the gray matter of the central nervous system 

 (see Fig. 4.9, p. 96). When nerve fibers are bound together and surrounded 

 by fibrous connective tissue, they form the visible nerves of the peripheral 

 nervous system (Fig. 3. 18/)); nerve fibers also make up the white matter of 

 the central nervous system. A nerve fiber is always continuous with the 

 cytosome of a neuron. 



Organs. The tissues that have been described illustrate the various types 

 of specialization that cells undergo in the vertebrate body. Particular tissues 

 are capable of performing their special functions alone, but they usually occur 

 grouped in organs. Thus, organs are groups of tissues associated for the per- 

 formance of a special function. For example, if the wall of the small intestine 

 of a mammal or other vertebrate is examined microscopically, it is found to 

 consist of layers known as the peritoneum, the longitudinal and circular muscle 

 layers, the submucosa, and the mucous membrane (Fig. 3.19). The peritoneum 



Longitudinal 



muscle layer Submucosa 



/„, .aa^ Capillary 



Peritoneum 



Circular 



muscle 



layer 



Muscularis 

 mucosae 



Lymphatic 

 vessel 



Goblet cell 



Fig. 3.19. The wall of the small intestine of an amphibian, in section; semidiagrammatic. 



69 



