HISTOLOGY 149 



rounded by a relatively small mass of cytoplasm which is pro- 

 longed into two or more processes of varying lengths, the Jiei-ve 

 fibers. Where there are only two cytoplasmic extensions, the cell is 

 called a bipolar neurone (Fig. 82 A and B). The process by which 

 nervous impulses travel toward the cell body is called a dendrite, 

 while the one over which impulses pass away from the cytosome 

 is the axon. There is never more than one axon, but there may be 

 many dendrites. Where there is more than one dendrite, the cell is 

 multipolar (Fig. 82 C). The cell bodies of neurones are found in 

 groups, forming ganglia, outside the central nervous system; 

 they also constitute the gray matter of the central system. 

 Nerve fibers, bound together and surrounded in bundles by fibrous 

 connective tissue, form the visible nerves of the peripheral nervous 

 system (Fig. 82 D) and make up the white matter of the central 

 nervous system. Certain nerve fibers are intimately covered by a 

 layer of mj^elin, which contains much lipin, and are known as 

 medullated fibers (Fig. 82 E). A nerve fiber is always a process of 

 a neurone. As will be recalled, neurones are named according to 

 their position in the reflex arc, but these names do not indicate 

 subdivisions of the main class. 



Organs and Systems. — The tissues that have been described 

 exemplify the various types of specialization that cells undergo in 

 th3 vertebrate body. Each particular tissue is capable of perform- 

 ing its special function alone, but they usually occur grouped in 

 organs. Thus, organs are groups of tissues associated together for 

 the performance of a special function. For example, if the wall 

 of the small intestine is examined microscopically, it is found to 

 consist of layers known as the peritoneum, longitudinal and circular 

 muscle layers, submucosa, and mucous membrane (Fig. 83). The 

 peritoneum consists of simple squamous epithelium and functions 

 as a covering membrane. Both longitudinal and circular muscle 

 layers are of non-striated muscle tissue, bound together by fibrous 

 connective tissue, and their contractions produce the muscular 

 movements that mix the food contents of the intestine and push 

 them along toward the lower parts of the tract (cf. Fig. 48, p. 84). 

 Fibrous connective tissue, containing both white and elastic fibers, 

 is the distinguishing tissue of the submucosa and serves to support 

 the numerous vessels carrying blood and lymph. This layer also 

 provides the elasticity necessary for the expansion of the canal, in 

 addition to carrying the circulatory fluids necessary for absorption. 



