THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 405 



glionic enlargement, the siil) esophageal ganglion, and encircle the 

 alimentary canal to join on the dorsal side as the pair of siipra- 

 esophageal ganglia or ''brain." In Arthropoda the ganglia of the 

 thorax have undergone considerable fusion. In Echinodermata, the 

 starfish for example, the central group of ganglia makes up the cir- 

 cumoral nerve ring around the mouth, and radial branches extend 

 into each arm. Branches from these communicate with the sensory 

 structures of the skin and tube feet. 



Concentration of the tissue of the nervous system into definite 

 organs is carried farther in vertebrates than in the less highly or- 

 ganized forms. The fact that the central nervous system of verte- 

 brates is dorsally located and hollow has been brought out previously. 

 Even within the group of vertebrates, the nervous system shows a 

 progressive increase in complexity. The highly developed brain of 

 the mammal is the climax of this tendency. 



The neurones have been referred to before as the units of structure 

 and function of the higher type of nervous system, from worms to 

 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- 



