GENERAL PHYSIOLOGY OF NERVE-TISSUE. 115 



PHYSIOLOGIC PROPERTIES OF NERVES. 



Nerve Irritability or Excitability and Conductivity. These 

 terms are employed to express that condition of a nerve which enables 

 it to develop and to conduct nerve impulses from the center to the 

 periphery, from the periphery to the center, in response to the action 

 of stimuli. A nerve is said to be excitable or irritable so long as it 

 possesses these capabilities or properties. For the manifestation of 

 these properties the nerve must retain a state of physical and chemic 

 integrity; it must undergo no change in structure or chemic composi- 

 tion. The irritability of an efferent nerve is demonstrated by the 

 contraction of a muscle, by the secretion of a gland, or by a change 

 in the caliber of a blood-vessel, whenever a corresponding nerve is 

 stimulated. The irritability of an afferent nerve is demonstrated by 

 the production of a sensation or a reflex action whenever it is stimu- 

 lated. 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 histologic 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 muscles, without impairing the irrita- 

 bility of either nerve-trunks or muscles. Atropin induces complete 

 suspension of glandular activity by impairing the terminal organs of 

 the secretory nerves just where" they come into relation with the 

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

 nerve;" 



Terve-fibers 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 



