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TEXT-BOOK OF PHYSIOLOGY 



FIG. 45. MOTOR NERVE-ENDINGS 

 OF INTERCOSTAL MUSCLE-FIBERS OF 

 A RABBIT. X 150. (Slohr.) 



roots contain all the efferent fibers, the posterior or dorsal roots all the afferent 



fibers. 



The Peripheral Endings of Nerves. The efferent nerves as they 



approach their ultimate terminations lose both the neurilemma and myelin 



sheaths. The axon or axis-cylinder then divides into a number of branches 



which become directly and intimately associated with tissue-cells. The 



particular mode of termination varies in 

 different situations. Tin-so terminations 

 are generally spoken of as end-organs, 

 terminal organs, or end-tufts. 



In the skeletal muscle the nerve-fiber 

 loses both neurilemma and myelin sheath 

 at the point where it comes in contact with 

 the muscle-fiber. After penetrating the 

 sarcolemma, the axon or axis-cylinder 

 divides into a number of small branches 

 which appear to be embedded in a rela- 

 tively large mass of sarcoplasm and nuclei, 

 the whole forming the so-called "motor 

 plate." Each muscle-fiber possesses one 

 such plate or end-organ in mammalia, 

 several in the frog. (Fig. 45.) 

 In the visceral muscle the terminal nerve-fibers derived from sympathetic 



or peripheral neurons are primarily non-medullated. The axons divide and 



subdivide and form plexuses which surround the muscle-cell bundles. Fine 



fibers from the plexuses are given off which ultimately come into relation 



with each individual cell, on the surface of which they terminate in the form 



of one or more granular masses. 



In the glands, taking as an illustration the parotid and mammary glands, 



the nerve-fibers, also derived from sympathetic or peripheral neurons, pass 



into the body of the gland and ultimately 



reach the acini, on the outer surface of 



which they ramify and form a plexus. 



From this plexus fine fibers penetrate 



the acinus wall and end on the gland-cell. 



The fibers present a varicose appearance 



(Fig. 46). 



The Receptor Substance. The 



points of contact of nerve and muscle- 



tissues, nerve and secretor cells have 



been termed the myoneural and cytoneural junctions. 



Experimental evidence has led to the view that the terminals of the 

 axon and the muscle-fiber, the terminals of the axon and the cell-substance 

 though histologically and physiologically related are not in absolute con- 

 tact but that there intervenes between nerve and muscle, the nerve and cell 

 a third substance differing from both and which serves to transfer the 

 energy of the nerve to the muscle and to the cell. This third substance has 

 been termed by Langley the "receptor substance." This substance it is 

 that apparently is acted on, either in the way of excitation or depression, 

 followed by increase or decrease in the activity of muscle-fiber or gland- 



B 



FIG. 46. TERMINATIONS OF NERVE- 

 FIBERS IN THE GLAND-CELLS. A. Cell of 

 the parotid gland of a rabbit. B. Cells 

 of the mammary gland of a cat in gesta- 

 tion. (Day on and 



