METAZOAN ORGANIZATION 



107 



this tissue is rather difficult, but the fibers are faintly segmented by 

 thin intercalary disks which define areas each with a single nucleus. 

 The cells branch laterally to join each other quite frequently, pro- 

 ducing a condition of netlike branching known as anastomosis. 



Nervous Tissue. — This is specialized to receive stimuli and trans- 

 mit impulses which have been set up by some stimulating agent in 

 some part of the body. The structural features consist of nerve 

 cell bodies and their processes. Two kinds of processes are recog- 

 nizable : (a) the axo7ie, usually a single unbranched fiber except for 

 infrequent collateral branches; and (b) dendrites, frequently much 



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Fig. 45. — Typical cells and tissues from vertebrate animals. 1, Squamous 

 epithelial cells ; Z, section through a portion of bone showing Haversian canal 

 (in center), bone cells, lacunae, canaliculi, and matrix; 3, section of hyaline 

 cartilage showing cartilage cells in lacunae, and matrix between lacunae ; i, sec- 

 tion of tendon composed of white fibrous connective tissue ; 5, longitudinal view 

 of smooth (involuntary) muscle cells; 6, striated (voluntary) muscle; 7^ motor 

 nerve cell, showing process; 8, human red blood (nonnucleated) corpuscles and 

 human white (nucleated) corpuscles. (Drawn by Titus Evans.) 



branched and arborlike. An axone may be several feet long, e.g., one 

 extending from the spinal cord to the hand or foot. Dendrites may 

 be lacking. The impulses are conducted toward the cell body over 

 the dendrites and away over the axone. A nerve cell body together 

 with its processes is called a neuron. The neurons approach each 

 other and pass impulses from one to the other at the synapses, where 



