182 ESSENTIALS OF ZOOLOGY 



man. Each neurone is a nerve cell with processes extending from it, 

 and each of these units must conduct nerve impulses in its normal 

 function. The exact nature of the nerve impulse is still somewhat 

 of a question. It is thought to be transmitted as a metabolic change 

 passing along the nerve fiber (axone or dendrite). This is at least 

 partially a chemical change in which oxygen is necessary and a cer- 

 tain amount of carbon dioxide is produced, but since there is only 

 slight increase in temperature during the change, it seems not to be 

 a typical metabolic oxidation process ; furthermore, the activity seems 

 not to fatigue the nerve fiber. An electrical charge follows the wave 

 of activity along the nerve fiber, but it apparently accompanies the 

 impulse or is a result of it rather than the impulse itself. The speed 

 of electrical transmission has been measured in a number of different 

 animals and nervous transmission is much slower than electrical. At 

 room temperature the sciatic nerve of a frog will transmit a nerve 

 impulse at the rate of about 100 feet per second. Conduction over 

 nonmedullated fibers of invertebrates is much slower than this. On 

 the other hand, measurements of the rate of conduction in man show 

 a velocity of about 400 feet per second. 



The reflex arc and reflex actions illustrate the simple form of nerv- 

 ous conduction circuit. In its simplest form the reflex arc is com- 

 posed of one motor and one sensory neurone ; however, it is usually 

 more complex. The classical example involves the spinal cord and 

 a spinal nerve. This is known as a reflex of the first level, because 

 it returns the motor impulse over the motor fibers of the same nerve 

 which brought in the sensory impulse. The motor axone carrying 

 the impulse from the motor nerve cell in the gray matter usually 

 ends in a muscle cell or a gland. There is no protoplasmic union 

 between the axone of the sensory neurone and the dendrite of the 

 motor, for these come in contact only by a synapse which brings them 

 in close proximity. It has been found experimentally that nervous 

 impulses may be conducted in either direction by the fiber but can 

 cross a synapse only from axone to dendrite, thus serving like a valve 

 in a pipeline. Keflex actions may be in the form of motion, as with- 

 drawal from unexpected pain, or shivering or formation of goose 

 flesh, or the contraction of the pupil of the eye with increased light 

 intensity. Still other reflex actions include secretion by glands, 

 breathing, movements of speech, individual actions included in walk- 

 ing, and others, 



