126 



THE VERTEBRATE ANIMAL: IRRITABILITY 



transmission of a nervous impulse along the afferent neurones asso- 

 ciated with the area. Nerves were at first thought to be tubes 

 that conducted the " animal spirits," which were supposed to be 

 like gas. Later the material carried by the tube was believed to be 

 more like water and was called " animal juice." The nature of the 

 nervous impulse and its transmission still remains a puzzle in many 

 respects and engages the attention of many investigators. Some 

 workers have believed that the transmission of the nervous impulse 

 is a wave of chemical reactions along the nerve fiber. The analogy 



Fig. 69. — A, Cells of olfactory epithelium from human nose. B, Cells 

 of a taste bud in epithelium of tongue. In both cases the sensory cells ter- 

 minate externally in hair-like processes that are able to receive the chemical 

 stimuli of olfaction or smell, and gustation or taste. 



ep, epithelium of tongue; yc, gustatory cell; oc, olfactory cell; p, pore or external 

 opening of epithelial cup in which the taste bud is located; sc, sustentacular or supportinj^ 

 non-sensory cell. 



to the burning of a path of gunpowder is often made. This idea 

 involves metabolic activities of a destructive kind in the neurone 

 process. Such metabolism uses oxygen and liberates carbon 

 dioxide and heat. It also is limited by the amount of combining 

 constituents of the protoplasm. The production of carbon dioxide 

 and heat in sufficient quantities to justify a chemical interpreta- 

 tion of transmission has not been demonstrated. The fact that it is 

 almost impossible to detect signs of fatigue, or loss of capacity to 

 transmit in a bundle of nerve fibers, is also an argument against the 

 metabolic theory. Opposed to the chemical interpretation is the 

 physical theory, which explains transmission as the passage of an 

 electric current. The velocity of nervous transmission varies from 

 27 to 125 meters per second in the cases measured. This is, of 



