82 HUMAN PHYSIOLOGY. 



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 as 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 compo- 

 sition. 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 efferent nerve is demon- 

 strated by the 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, 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. In cold-blooded animals, on the contrary, in 

 which the nutritive changes take place relatively slowly, the irrita- 

 bility 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 modify- 

 ing or depressing the conductivity of the end organs of the nerves 

 just where they come in contact with the muscles without impairing 

 the irritability of either nerve trunks or muscles. Atropin induces 

 complete suspension of glandular 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 or nerve. 



