MEDULLA OBLONGATA OF AMBLYSTOMA 383* 



sensori-motor apparatus, but each of the larger correlation neu- 

 rones (and many of the peripheral motor neurones also) seems to 

 effect synaptic relations with the entire stratum album, to wit, 

 with sensory root fibers of all kinds and also with all of the long 

 correlation tracts of the cord. In the mammalian medulla 

 oblongata, on the other hand, each functional component of the 

 various sensory roots of the cranial nerves terminates in a specially 

 differentiated end-nucleus, from which neurones of the second 

 order conduct the nerve impulses of their respective sensory types 

 either directly to the motor centers through the reticular formation 

 or by long tracts to higher correlation centers. These secondary 

 tracts transmit unmixed specific "sensory excitations of the same 

 functional type as the roots with which they are functionally 

 related. Finally, in the oblongata of urodele larvae we find a 

 third type of correlation neurones. The peripheral neurones of 

 the sensory roots of the cranial nerves exhibit a specificity of 

 function which appears to be as precisely localized as that of the 

 corresponding mammalian nerves and to show essentially the 

 same arrangement of nerve components in the cranial roots. The 

 regional localization of the end-stations of these various func- 

 tional types of root fibers is tolerably precise and clearly defined. 

 But the sensory neurones of the second order are not arranged in 

 similarly circumscribed nuclei related respectively to these definite 

 end-stations of the peripheral neurones. On the other hand, prac- 

 tically all of the neurones of the adjacent gray substance send their 

 dendrites into at least two and often into several different end- 

 stations, so that the same secondary neurone may be, and appa- 

 rently habitually must be, capable of excitation by two or more 

 diverse end-organs. Thus, a single neurone of the oblongata may 

 effect synaptic connection with glossopharyngeal fibers from taste 

 buds and also with trigeminal fibers of tactile sensibility (fig. 38), 

 with root fibers from the V and also from the VIII cranial nerves 

 (fig. 31), or with fibers of the tactile sense coming in from the head 

 by way of the V cranial nerve and with fibers of the tractus spino- 

 bulbaris conveying tactile impulses from the trunk region (figs. 

 21, 22, 23, 24, 25). Each sensory neurone of the second order in 

 the oblongata is, accordingly, at the same time a correlation neu- 



