Johnston, Morphology of the Head. 179 



from the mucosae enter, we could not assume that the observed 

 distinct peripheral course of these two sets of fibers had any- 

 functional significance. If it had proved that the fibers from 

 the skin and those from the mucosae entered the same centers 

 and had the same primary and secondary connections within 

 the brain, we should have been compelled to conclude that sen- 

 sory impulses from the skin and those from the mucosae would 

 bring about the same reactions. As a matter of fact it has 

 proved not only that the primary centers of these two sets of 

 fibers are distinct, but that their secondary and tertiary fiber 

 tracts remain separate and distinct. Hence it may be concluded 

 that any fiber bringing impulses into the cutaneous center must 

 set up reactions which characteristically follow cutaneous stimu- 

 lation. The same reasoning may and must be applied to each 

 system of nerve components. Furthermore, the central relations 

 of any system of nerve components when certainly known serve 

 as a starting point from which one may reason back with perfect 

 validity to the homology and probable function of the organs in- 

 nervated by that system. To illustrate by a case which is 

 still under some dispute, it was by this method that the writer 

 concluded that the end buds belonged to the visceral sensory 

 system before their gustatory function had been proved by ex- 

 tended experiment {6"] , 49). 



Second, the brain sometimes gives clues to primitive func- 

 tional relationships of which the peripheral nerves no longer 

 bear traces. In certain segments of the head this or that nerve 

 component is wholly lacking owing to the disappearance of the 

 structure which it originally innervated. Yet in these same 

 segments the brain retains in some cases the center or column 

 which this component should enter. A simple example of 

 this is the presence of the somatic motor column throughout 

 the medulla oblongata even in those forms in which several 

 segments are without any other vestige of ventral motor 

 nerves. The cases of the cerebellum and tectum opticum have 

 been considered at some length in previous papers {6'] , 68, 69, 

 70), and these and other cases of the same sort will be treated 

 further in the present paper. Wherever the brain contains a 



