62 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



fibrils is an important observation from the physiological point of view; see 

 Circulation Chapter. 



In certain parts of the heart, the cardiac tissue is not completely differ- 

 entiated and retains in the adult somewhat embryonic characters; for ex- 



FIG. 79- 



FIG. 80. 



FIG. 79. Muscular Fiber Cells from the Heart. (E. A. Schafer.) 



FIG. 80. From a Preparation of the Nerve Termination in the Muscular Fibers of a Snake. 

 a, End plate seea o.ily broad-surfaced: b, end plate seen as narrow surface. (Lingard and Klein.) 



ample, the bundle of His running in the septum from the auricles to the 

 ventricles and the cells containing Purkinje's fibers lying immediately under 

 the endocardium. 



Blood and Nerve Supply. The muscles are freely supplied with 

 blood-vessels; the capillaries form a network with oblong meshes around 

 the fibers. Nerves also are supplied freely to muscles; the striated voluntary 

 muscles receiving them from the cerebro-spinal nerves, and the cardiac 

 muscle from both the cerebro-spinal and the sympathetic nerves. 



In striped muscle the nerves end in motor end-plates. The nerve fibers 

 are medullated; and when a branch passes to a muscle fiber, its primi- 

 tive sheath becomes continuous with the sarcolemma, and the axis-cylinder 

 forms a network of its fibrils on the surface of the muscle fiber. This net- 

 work lies embedded in a flattened granular mass containing nuclei of several 

 kinds; this is the motor end-plate, figures 80 and 81. There is considerable 

 variation in the exact form of the nerve end-plate in the muscle. In 

 batrachia the nerve fiber ends in a brush of branching nerve fibrils which 

 are accompanied here and there by attached oval nuclei. 



Development. The striated muscle of the voluntary variety is 

 usually developed from the mesoderm. The embryonic cells increase enor- 

 mously in size, the nuclei multiply by fission and distribute themselves be- 

 neath the sarcolemma. There is a differentiation of the cell protoplasm 



