GENERAL PHYSIOLOGY OF NERVE-TISSUE. 99 



production of a sensation or a reflex action whenever it is stimulated. The 

 irritability of nerves continues for a certain period of time after separation 

 from the nerve-centers and even after the death of the animal, the time 

 varying in different classes of animals. In the warm-blooded animals, in 

 which the nutritive changes take place with great rapidity, the irritability 

 soon disappears a result due to disintegrative changes in the nerve, caused 

 by the withdrawal of the blood-supply and other non-physiologic conditions. 

 In cold-blooded animals, on the contrary, in which the nutritive changes 

 take place relatively slowly, the irritability lasts, under favorable conditions, 

 for a considerable time. Other tissues besides nerves possess irritability, 

 that is, the property of responding to the action of stimuli e.g., glands and 

 muscles, which respond by the production of a secretion or a contraction. 



Independence of Tissue Irritability. The irritability of nerves is 

 distinct and independent of the irritability of muscles and glands, as shown 

 by the fact that it persists in each a variable length of time after their histo- 

 logic connections have been impaired or destroyed by the introduction of 

 various chemic agents into the circulation. Curara, for example, induces 

 a state of complete paralysis by modifying or depressing the conductivity of 

 the end-organs of the nerves just where they come in contact with the mus- 

 cles, without impairing the irritability of either nerve-trunks or muscles. 

 Atropin induces complete suspension of gland activity by impairing the 

 terminal organs of the secretor nerves just where they come into relation with 

 the gland-cells, without destroying the irritability of either gland-cell or nerve. 



Nerve Stimuli. Nerves do not possess the power of spontaneously 

 generating and propagating nerve impulses; they can be aroused to activity 

 only by the action of an external stimulus. In the physiologic condition the 

 stimuli capable of throwing the nerve into an active condition act for the 

 most part on either the central or peripheral end of the nerve. In the case 

 of motor nerves the stimulus to the excitation, originating in some molecular 

 disturbance in the nerve-cells, acts upon the nerve-fibers in connection with 

 them. In the case of sensor or afferent nerves the stimuli act upon the pecul- 

 iar end-organs with which the sensor nerves are in connection, which in 

 turn excite the nerve-fibers. Experimentally, it can be demonstrated that 

 nerves can be excited by a sufficiently powerful stimulus applied in any 

 part of their extent. 



Nerves respond to stimulation according to their habitual function; 

 thus, stimulation of a sensor nerve, if sufficiently strong, results in the sensa- 

 tion of pain; of the optic nerve, in the sensation of light; of a motor nerve, 

 in contraction of the muscle to which it is distributed; of a secretor nerve, 

 in the activity of the related gland, etc. It is, therefore, evident that pecul- 

 iarity of nerve function depends neither upon any special construction or 

 activity of the nerve itself nor upon the nature of the stimulus, but entirely 

 upon the peculiarities of its central and peripheral end-organs. 



Nerve stimuli may be divided into 



1. General stimuli, comprising those agents which are capable of exciting 



a nerve in any part of its course. 



2. Special stimuli, comprising those agents which act upon nerves only 



through the intermediation of the end-organs. 

 The end-organs are specialized highly irritable structures placed between 



