IRRITABILITY OF NERVES. 629 



has been found that during rest the chromatic substance accumulates in the 

 cells, while it is consumed during activity. Actively functionating cells are 

 enlarged, as are also their nuclei and nucleoli. Exhaustion is indicated by 

 contraction of the nucleus, probably also of the cell, and by the formation of 

 a diffusely staining substance in the nucleus. The clearing up in the vicinity of 

 the nucleus is due to disappearance of the chromatic substance. According to 

 Levi numerous granules appear in the chromatic substance in rabbits as a result 

 of activity of the spinal ganglia. These granules were wanting in the state of 

 rest. Pick found the Nissl bodies in the spinal cord reduced in size. Demoor 

 maintains that the active cells of the visual area stain less intensely, are diminished 

 in size, and have an irregular nucleus. Poisoning with morphin gives rise to a 

 granular condition of the protoplasmic processes of the cortical cells. The cells 

 of the motor area of the cortex are shrunken after long-continued irritation, and 

 their processes are granular. 



IRRITABILITY OF NERVES. STIMULI. 



Nerves possess the property of being thrown into a condition of 

 irritability by stimuli, and they are, therefore, spoken of as irritable. 

 Stimuli may be effective if applied at any point in the course of a nerve. 

 An entirely uninjured and normally nourished nerve possesses the same 

 degree of irritability at all points in its course. In new-born animals 

 and in human beings up to the sixth week the nerves (and muscles) 

 react less readily to electrical stimuli; the resulting contractions are 

 slower and more extensive. The cause appears to reside in the incom- 

 plete development and evolution of these tissues. All stimuli, if power- 

 ful and long continued, soon cause paralysis by over-irritation of the nerve 

 at the site of application. The nerve, therefore, loses its irritability at 

 this point. Further on, toward the periphery, however, its irritability 

 is still retained. 



Mechanical stimuli affect the nerve when they induce a change in the form 

 of the nerve-particles with a certain degree of rapidity; for example, a blow, 

 pressure, crushing, traction, puncture, section, concussion, sudden release of ten- 

 sion. In the case of sensory nerves pain occurs as a result ("falling asleep" of 

 the extremities ; pain on striking the ulnar nerve in its groove at the elbow) ; 

 in the case of motor nerves, muscular contraction. If the mechanical injury to 

 the fibers has resulted in interference with the continuity of their conducting 

 elements (the axis-cylinders), the conductivity of the nerve ceases. If the 

 molecular arrangement of the nerve-particles is permanently disturbed (for 

 example by concussion) , the irritability of the nerve is lost. 



A light blow on the musculo-spiral nerve in the arm, on the brachial plexus 

 in the supraclavicular fossa, causes in normal individuals contraction in the muscles 

 supplied. The mechanical irritability of nerves may be abnormally increased 

 under pathological conditions. 



Tigerstedt discovered that the minimal value of the mechanical stimulation 

 (induced by the falling of a weight upon the isolated nerve) is 900 milligram- 

 millimeters, the maximal value from 7000 to 8000. More powerful stimulation 

 causes exhaustion, but this does not extend beyond the irritated area. The 

 mechanically irritated nerve does not acquire an acid reaction. A lesser degree 

 of pressure or tension increases the irritability, which again diminishes after a 

 short time. The work done by the irritated muscle as a result of this irritation 

 was as much as 100 times greater than the kinetic energy of the mechanical nerve- 

 irritation. 



If a mechanical influence acts gradually the nerve may lose its conductivity 

 or its irritability without any manifestation of irritation in the process. This class 

 of phenomena includes the paralysis in the distribution of the brachial plexus as 

 a result of long-continued pressure by a crutch and the paralysis of the recurrent 

 laryngeal nerve by aneurysms. 



As a result of pressure upon nerves by gradually increasing the weight applied 

 there was observed at first a.n increase and then a diminution in the irritability. 



