CH. viu.] IRRITABILITY. IO5 



may have many branches, only gives off one process which becomes 

 the axis cylinder of a nerve-fibre. This acquires a medullary sheath 

 when it passes into the white matter of the brain or spinal cord, 

 and a primitive sheath when it leaves the nerve-centre and gets into 

 the nerve. But at first the axis cylinder is not sheathed at all. 



The formation of the sheaths is still a matter of doubt, but 

 the generally accepted opinion is that the primitive sheath is 

 formed by cells which become flattened out and wrapped round 

 the fibre end to end. These are separated at the nodes by inter- 

 cellular or cement substance stainable by silver nitrate (fig. 121). 

 These cells are probably mesoblastic. The medullary sheath is 

 formed according to some by a fatty change occurring in the 

 parts of these same cells which are nearest to the axis cylinder, 

 but it is much more probable that it is formed from the peri- 

 pheral layer of the axis cylinder ; the presence of neurokeratin 

 in it distinctly points to an epiblastic origin. The fact also that, 

 in the nerve centres, the medullated nerve fibres have no primi- 

 tive sheath, and the phenomena of Wallerian degeneration, to be 

 described later, all tend to confirm the same view. 



CHAPTER VIII. 



IRRITABILITY AND CONTRACTILITY. 



Irritability or Excitability is the power that certain tissues 

 possess of responding by some change to the action of an external 

 agent. This external agent is called a stimulus. 



Undifterentiated cells like white blood corpuscles are irritable ; 

 when stimuli are applied to them they execute the movements we 

 have learnt to call amoeboid. 



Ciliated epithelium cells and muscular fibres are irritable ; they 

 also execute movements under the influence of stimuli. 



Nerves are irritable ; when they are stimulated, a change is 

 produced in them ; this change is propagated along the nerve and 

 is called a nervous impulse ; there is no change of form in the 

 nerve visible to the highest powers of the microscope ; much 

 more delicate and sensitive instruments than a microscope must 

 be employed to obtain evidence of a change in the nerve ; it is 

 of a molecular nature. But the irritability of nerve is readily 

 manifested by the results the nervous impulse produces in the 

 organ to which it goes ; thus the stimulation of a motor nerve 

 produces a nervous impulse in that nerve which, when it reaches 



