MECHANICAL STEMULI AND NERVE-STRETCHING, 533 



while occlusion of the cerebral vessels causes almost instantaneously cessation of 

 the cerebral functions. The metabolism of the central nervous organs is much 

 more active than that of the nerves themselves. [If the abdominal aorta of a 

 rabbit be compressed for a few minutes, the hind limbs are quickly paralysed, the 

 animal crawls forward on its fore-legs, drawing the hind limbs in an extended 

 position after it.] The ganglia form much lymph. 



324. EXCITABILITY OF THE NERVES -STIMULI. Nerves possess the 

 property of being thrown into a state of excitement by stimuli, and are, therefore, 

 said to be excitable or irritable. The stimuli may be applied to, and may act 

 upon, any part of the nerve. [The following are the various kinds of stimuli, i.e., 

 modes of motion, which act upon nerves] : 



1. Mechanical stimuli act upon nerves when they are applied with sufficient 

 rapidity to produce a change in the form of the nerve-particles, e.g., a blow, 

 pressure, pinching, tension, puncture, and section. In the case of sensory nerves, 

 when they are stimulated, pain is produced, as is felt when a limb "sleeps," or 

 when pressure is exerted upon the ulnar nerve at the bend of the elbow. When a 

 motor nerve is stimulated, motion results in the muscle attached to the nerve. If 

 the continuity of the nerve-fibres be destroyed, or, what is the same thing, if the 

 continuity of the axial cylinder be interrupted by the mechanical stimulus, the 

 conduction of the impulse across the injured part is interrupted. If the molecular 

 arrangements of the nerves be permanently deranged, e.g., by a violent shock, the 

 excitability of the nerves may be thereby extinguished. 



A slight blow applied to the radial nerve in the fore-arm, or to the axillary nerves in the 

 supraclavicular groove, is followed by a contraction of the muscles supplied by these nerves. 

 Under pathological conditions, the excitability of a nerve for mechanical stimuli may be 

 increased enormously. 



Tigerstedt ascertained that the minimal mechanical stimulus is represented by 900 milligram- 

 millimetres, and the maximum by 7000 to 8000. Strong stimuli cause fatigue, but the fatigue 

 does not extend beyond the part stimulated. A nerve when stimulated mechanically does not 

 become acid. Slight pressure without tension increases the excitability, which diminishes after 

 a short time. The mechanical work produced by an excited muscle in consequence of a stimulus 

 was 100 times greater than the mechanical energy of the mechanical nerve-stimulus. 



Continued pressure upon a mixed nerve paralyses the motor sooner than the 

 sensory fibres. If the stimulus be applied very gradually, the nerve may be 

 rendered inexcitable without manifesting any signs of its being stimulated 

 (Fontana, 1758). Paralysis, due to continuous pressure gradually applied, may 

 occur in the region supplied by the brachial nerves ; the left recurrent laryngeal 

 nerve also may be similarly paralysed from the pressure of an aneurism of the arch 

 of the aorta. 



By increasing the pressure on a nerve by using a gradually increasing weight, there is at first 

 an increase and then a decrease of the excitability. Pressure on a mixed nerve abolishes reflex 

 conduction sooner than motor conduction {Kronecker and Zederbaum). 



Nerve-stretching is employed for therapeutical purposes. If a nerve be exposed and 

 stretched, or if it be made sufficiently tense, the nerve is stimulated. Slight tension increases 

 the reflex excitability (Schleich), while violent extension produces a temporary diminution or 

 abolition of the excitability (Valentin). The centripetal or sensory fibres of the sciatic nerve 

 are sooner paralysed thereby than the centrifugal or motor (Conrad). During the process of 

 extension, mechanical changes are produced, either in the nerve itself or in its end-organs, 

 causing an alteration of the excitability, but it may also affect the central organs. The 

 paralysis, which sometimes occurs after forcible stretching, usually rapidly disappears. There- 

 fore, when a nerve is in an excessively excitable condition, or when this is due to an inflam- 

 matory fixation or constriction of the nerve at some part of its course, nerve-stretching may 

 be useful, partly by diminishing the excitability, partly by breaking up the inflammatory 

 adhesions. In cases where stimulation of an afferent nerve gives rise to epileptic or tetanic 

 spasms, nerve-stretching may be useful by diminishing the excitability at the periphery, in 

 addition to the other effects already described. It has also been employed in some spinal 

 affections, which may not as yet have resulted in marked degenerative changes. 



For physiological purposes, a nerve , may be stimulated mechanically by means of Heiden- 



