METAZOAN ORGANIZATION 57 



There are three types of muscular tissue : smooth, involuntary, and 

 nonstriated, as found in the wall of the intestine; striated, volun- 

 tary, skeletal, as found in the muscle of the arm ; and striated, in- 

 voluntary, cardiac, as found in the wall of the heart. Skeletal, 

 voluntary muscle is made up of large multinucleate (many nuclei) 

 fibers, each composed of many fibrils (myofibrils) along which are 

 evenly distributed dense and light areas, giving the general ap- 

 pearance of stripes across the cell, because the dense areas on the 

 adjacent fibrils come at the same level. The smooth involuntary 

 muscle is composed of individual, spindle-shaped (fusiform) cells, 

 the cytoplasm of which is largely myofibrils but without striations 

 and therefore smooth. There is a single oval nucleus, centrally lo- 

 cated. The outer membrane of a muscle cell is the sarcolemma. 

 The cardiac involuntary muscle is said to be made up of individual 

 cells, highly modified in arrangement. The definition of cells in 

 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 axoncy usually a single unbranched fiber except for 

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

 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 

 the brushlike ending of the axone of one comes into close proximity 

 with a dendrite of another. In this way an impulse can be trans- 

 mitted from one part of the body to other parts. The chief function 

 of the nervous tissue is to relate the organism to its environment. 



Vascular Tissue. — This is fluid tissue consisting of cells known as 

 corpuscles in a fluid medium called plasma. The cells are the red 

 corpuscles (erythrocytes) and white corpuscles (leucocytes), while 

 the plasma or fluid is the intercellular substance. Blood and lymph 



