different muscles, as it ranges from O'l to O'Ol mm. The fibres are 

 cylindrical or prismatic in form, with rounded angles and conical 

 ends. They consist of a striated substance of soft consistency 

 (the structure of which we shall presently examine) enclosed in 

 a tubular, apparently homogeneous elastic sheath, called the 

 sarcolemma ; this is continued at both ends of the fibre into 

 connective tissue fibrils, which join the tendon or the septa of 

 the perimysium. 



The muscle fibres alone become active when the muscle 

 contracts ; the sarcolemma, the connective tissue of the perimysium 

 and its intermuscular septa, and the tendons remain passive. 

 During contraction each fibre pulls upon the tendon, either 

 directly or by means of the interfascicular connective tissue 

 which is continued into the tendon. 



Each muscle has medullated and non-medullated nerve fibres ; 

 the former innervate its fibres, the latter the walls of the blood- 

 vessels : every muscle fibre is provided with at least one nerve 

 fibre, which usually forms an end-plate near its middle. 



Under normal conditions the skeletal muscles are thrown 

 into activity by their nerves, and after section of these all move- 

 ment of the muscle is arrested ; this indicates that neither the 

 muscles nor the nerves by which they are innervated are capable 

 of automatic activity. But after dividing the nerve and exposing 

 the muscle an effective mechanical, thermal, chemical or electrical 

 stimulus, applied either to the nerve or directly to the muscle, 

 evokes a contraction of the latter in response ; so that both 

 nerves, when severed from their centre, and voluntary muscles 

 manifest irritability or excitability, i.e. a power of reacting by an 

 explosion of energy to external impulses (Vol. I. p. 44). The 

 active reaction, or contraction, of the muscle is expressed in its 

 rapid change of form and displacement ; excitation of the nerve, 

 on the contrary, is not accompanied by any direct visible change, 

 and consists solely, as we shall see, in a molecular vibration, by 

 which the excitatory impulse is transmitted to the muscle. 



Since the natural excitation of a muscle is always the effect 

 of an excitation through its nerve, it is legitimate to assume that 

 the reaction produced artificially by its direct stimulation is also 

 due to stimulation of the nerve fibres that run between the 

 muscle bundles. Many authors, from Borelli and Willis onwards, 

 have regarded the -muscles as the passive instruments of the 

 nerves, though A. Haller maintained the opposite view in his 

 famous theory of muscular irritability, which was based on 

 fallacious arguments (Vol. I. p. 299). Although Haller's view 

 has now only an historical interest, it is instructive to sum up 

 briefly the most striking arguments that were, and still might be, 

 adduced in support of the theory of direct or autonomous excita- 

 bility of the voluntary muscles. 



