44 BRAINS OF RATS AND MEN 



or away from the source of the stimulus being almost 

 the only possible reactions. In other words, the 

 elaborately diversified receptor mechanisms converge 

 into very simply organized final common paths. Even 

 in the half-grown larva the motor apparatus of the 

 spinal cord seems to be arranged to serve chiefly total 

 reactions of the simplest locomotor type. The spinal 

 cord of larval Amblystoma may be compared with a 

 house provided with many entrances and but one 

 exit; that of a mammal with a house provided with 

 still more entrances and also a large number of exits. 



Primitively, the medulla oblongata was probably 

 organized much like the spinal cord just described, 

 nervous impulses from the various sense organs of 

 the head (tactile, gustatory, etc.) discharging into 

 a common nervous center. This center in higher 

 animals has been broken up into a number of separate 

 primary sensory centers, one for each of the sense 

 qualities received by this part of the brain. 



An interesting transitional stage in the develop- 

 ment of specificity of primary sensory centers is 

 found in the medulla oblongata of larval Amblystoma 

 (Herrick, 19 14). On the left side of Figure 2 we see 

 lateral line fibers {l.l.X.), vestibular fibers {n.VIII.)^ 

 and tactile fibers {gen. cut, X.) discharging into a 

 single correlation neuron, whose axon crosses to the 

 other side and enters a correlation tract {Im.). On 

 the right side of the figure a gustatory fiber {vise, 

 sens. X.) and a tactile fiber {gen, cut. X.) discharge 



