GENERAL STRUCTURE OF THE NERVOUS SYSTEM. 351 



FIG. 89. 





1 



passing beneath the sarcolemma, divides into a terminal arborization, 

 which lies in contact with the striated muscular surface. The stimulus 

 conveyed through a single nerve fibre is thus 

 communicated to the contractile substance of 

 the muscle at many different points. With the 

 exception of some variations of form in differ- 

 ent species, the details of the muscular termi- 

 nation of nerve fibres are essentially the same 

 in reptiles, birds, and mammalians. As the 

 ultimate nerve fibre reaches the point of its 

 attachment to the muscle, the sheath of Henle, 

 with which it was surrounded, leaves it and 

 becomes continuous with the sarcolemma. At 

 the same time its medullary layer terminates, 

 in the usual way, at an annular constriction. 

 That portion of the nerve fibre immediately 

 outside the sarcolemma is its last inter-annu- 

 lar segment ; arid within the sarcolemma the 

 axis cylinder is destitute of myeline. At this 

 situation the axis cylinder breaks up into its 

 terminal arborization ; and it is the form, 

 direction, and frequency of these ramifications 

 which constitute the main differences in this 

 respect between different animals. Each mem- 

 ber of the terminal arborization is surrounded 

 by a light zone of granular matter, in which 

 large flat oval nuclei, with well-marked nu- 

 cleoli, are imbedded. The only parts of the 

 nerve fibre therefore in immediate contact 

 with the contractile muscular substance are 

 those derived from the ramification of its axis 

 cylinder. 



Physiological Properties of the Nerve Fibres. 

 The nerve fibres are channels of communica- 

 tion between the nervous centres on the one 

 hand and the peripheral organs on the other. 

 For this purpose they are endowed with a 

 special irritability by which, when excited at 

 one end, they transmit the impulse throughout 

 their entire length, and produce an effect at 

 the opposite extremity. Those distributed to SENSJTIVE NFRVE fnaat 

 the skin, when excited at the periphery, pro- END-BULB; from the conjunc- 

 duce in the brain a corresponding sensation. 

 On the other hand, those distributed to the 

 muscles, when excited at their origin by the 

 impulse of the will, cause contraction in the 

 muscular fibres. This action produces no visible change in the nerve 





tiva of man. The turns of the 

 axis cylinder, within the bulb, 

 are exhibited in various trans- 

 verse and oblique sections. (Key 

 and Retzius.i 



