6o THOMPSON YATES LABORATORIES REPORT 
From these experiments it becomes obvious that if to the three divisions of the Vth 
cranial we apply as a test for complete segmental character the fact found to hold good for all 
complete spinal sensory nerves, i.e., that tlie skin-field of a complete segmental nerve extends con- 
tinuously from the mid-dorsal line of the animal to the mid-ventral line, the test gives, as regards 
the three divisions of the Vth cranial, a distinct reply. It is impossible to mark exactly the 
anterior polar point of the body, i.e., that point in the anterior end of the animal which occupies 
a position there similar to that held by the tip of the tail in the posterior end of the animal ; that 
being so, it is impossible to say whether the skin-field of the ramus ophthalmicus does really reach 
the mid-ventral line of the body ; it certainly occupies some of the mid-dorsal line, but the 
anterior pole of the body being the place of junction of the mid-dorsal with the mid-ventral line 
of the body, to admit ignorance of the position of the anterior polar point is tantamount to 
admitting that we do not know where the mid-dorsal line ends and the mid-ventral line begins. 
Not being able to say, therefore, whether the field of the ramus ophthalmicus does extend to the 
mid-ventral line, we cannot apply the test confidently to it. In the same way we cannot say 
whether the ramus maxillaris of the Vth extends to the mid-dorsal line, although we can be 
certain that it does extend to the mid-ventral line. The only conclusion that results from the 
application of the test to the skin-fields of these two nerves is, therefore, that if one of them is a 
complete segmental field, then the other cannot be, because there are not two anterior polar points 
to the body. 
But if either of these two fields is a complete segmental one, and is at the same time 
overlapped by the field of the Ilnd cervical nerve, then the remaining one of the two fields 
cannot be overlapped by the field of the Ilnd cervical ; at least, no such extent of overlapping is 
evidenced in other parts of the body. On the other hand, if the two fields be neither of them, 
when considered separately, complete segmental fields, but be both merely portions of one 
segmental field, each of them might be well overlapped by the complete segmental field behind, 
namely, that of the Ilnd cervical nerve. 
Examination of the fields shows that each is overlapped by the field of the Ilnd cervical 
nerve, though that of the ramus ophthalmicus constantly and to a greater extent than that of the 
ramus maxillaris, rarely and minutely overlapped. This part of the test indicates, therefore, that 
the combined skin-fields of the ist and and divisions of the trigeminus may be together equi- 
valent to the field of a complete posterior root of a segmental (spinal) nerve, but that neither of 
the fields is so when taken singly. 
But the skin-field supplied by the 3rd division of the nerve is, like tliose supplied by the 1st 
and 2nd divisions, overlapped from behind by that of the Ilnd cervical nerve, and to a far greater 
amount than they. This argues for the skin-fields of the first and third divisions of the trigemi- 
nus belonging to the same segmental level, and for no one of them by itself constituting a 
complete segmental area. Further, to the skin-field of the ramus mandihularis, the test for a 
complete segmental skin-field, namely, that it extends from mid-dorsal line without a break to the 
mid-ventral line, can be applied with certainty, since its position in regard to those lines is not 
dubious. The field, as a fact, occupies the mid-vcntral line along a considerable length ; but it 
