282 G. E. COGHILL 



glionic portion of the trigeminus is not functionally connected 

 with the motor centers, might explain the low degree of respon- 

 siveness to stimuli in this region. On the other hand, if the 

 responses are effected through the ganglionic system of the tri- 

 geminus their infrequency in the coiled-reaction stage must depend 

 upon the limitations of the zone of influence of the root fibers 

 within the brain, for the peripheral distribution of the nerve at 

 this time is very well developed. 



As a factor in this problem the descending trigeminal tract 

 naturally suggests itself. This tract has been described in the 

 first part of the paper as being of a composite nature — made up 

 of processes of neurones of the first and second, and possibly 

 higher orders, in varying proportions according to the region. 

 The root fibers of the trigeminal ganglia run some distance caudad 

 in the coiled-reaction stage — probably almost to the level of the 

 auditory vesicle. Beyond this the tract, now extending into the 

 spinal region, consists of processes from central neurones along 

 the course. If, then, stimuli reach the motor centers in the lower 

 regions of the medulla oblongata through the descending tri- 

 geminal tract they would have to traverse a series of relatively 

 short neurones with intercalated synapses after leaving the tri- 

 geminal conductors. Such a conduction path would certainly 

 introduce relatively great resistance if it did not block the stimuli 

 altogether. This is, in all probability, the really significant fac- 

 tor in determining the behavior of embryos of the coiled-reaction 

 stage to tactile stimulation of the preauditory region. 



In the early swimming stage the condition is quite different. 

 The trigeminal fibers now reach almost or quite to the level of 

 the vagus root, that is, into the immediate vicinity of the centers 

 that dominate the whole motor mechanism. Accordingly, re- 

 sponse to stimulation of the trigeminal region has become prompt 

 and regular. The data at hand, therefore, afford very good 

 evidence that such a descending trigeminal tract as occurs in 

 the early flexure stage (fig. 22) is at best a very inefficient con- 

 ductor; and that before response can become prompt and con- 

 stant the root fibers of the nerve must reach directly into the 

 approximate levels of the motor centers. We have in this con- 



