METABOLISM IN VERTEBRATES 



filtration through the walls of the capillaries and does not contain all the 

 constituents of blood plasma. Lymph is important as the pathway over 

 which pass the materials transferred between blood and the cells of the 

 body. 



Contractile Tissue. Contractile tissues, made up of what are known as 

 muscle cells, are of three kinds: non-striated, cardiac, and striated. The 

 cytoplasm of muscle cells is characterized by the presence of numerous fine 

 fibers, which are placed longitudinally. The shortening and thickening of 

 these muscle fibrillae result in the contraction of the individual cell and are 

 therefore responsible for the particular function of this kind of tissue, the 

 production of motion. Non-striated muscle cells are typically spindle-shaped 

 with the nucleus centrally placed (Fig. ?>.\1A and B). These cells usually 

 occur in sheets loosely held together by fibrous connective tissue. This kind 

 of muscle is found in the wall of the digestive tract, in the urinary bladder, 

 and in the walls of blood vessels. It is sometimes called involuntary muscle 

 because it is not under conscious nervous control. 



Cardiac muscle is found only in the heart and is capable of rhythmical 

 contractions (Fig. 3.17/^. The cells are arranged in the form of a syn- 

 cytium; that is, the cylindrical cytoplasmic units containing the nuclei are 

 not separated from one another by membranes where they meet at their ends. 



Fig. 3.17. Contractile tissues. A, non-striated muscle cells in cross section; note that the 

 cytosomes of many of the cells are not cut through in the region of the nucleus. B, isolated 

 non-striated muscle cell from the wall of the digestive tract, showing spindle-shaped cytosome 

 and single nucleus. C, cross section of a voluntary muscle; the striated muscle cells are held 

 together in bundles by fibrous connective tissue. D, portion of a striated muscle cell, showing 

 its multinucleate condition and the cross-striations; the cell has been injured near its right 

 end in order to show the cell membrane. E, striated muscle cells, showing blood supply. F, 

 cardiac muscle cells from the human heart. {A, B, C, and E from drawings by D. F. Robert- 

 son; F, from E. A. S. Schafer, Essentials of Histology, copyright 1916 by Longmans, Green 

 and Co., reprinted by permission.) 



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