EXAMINATION OF SOME SPINAL NERVES 57 
of the 3rd division of the nerve [nuiius nuindilmldrii) in so far accuratelv justifies the name 
mandibular. 
The field of cutaneous distribution of the trigeminus possesses particular interest, from 
Gaskell's suggestion that the huge sensory root of tlie nerve is compounded of a series of 
originally separate cranial sensory nerves secondarily massed together. The large area of the 
cutaneous field would then be really compounded of several segmental fields ; each of these it 
might be possible to separately determine by appropriate section of the other divisions of the nerve. 
The intra-medullary course of the nerve-fibres arising from the Gasserian ganglion has for some of 
them been traced down to the level of the Ilnd cervical segment of the spinal cord. As regards 
juxtaposition and overlap, it will be shown that the skin-fields of the Vth cranial and of Ilnd 
cervical mutually behave as if the two nerves were immediatelv juxtaposed members in a spinal 
series, without intercalation of any intermediate segment.* All that part of the skin-field 
of the Vth cranial which is overlapped by the field of the Ilnd cervical is, of course, segmentally 
posterior to the rest of the field. Of the large area not so overlapped it was interesting to inquire 
from the sensory cutaneous supply which part might be segmentally anterior, which posterior. 
The sensory fibres for even the conjunctiva and cornea are traceable to quite low, in the floor of 
the fourth ventricle, and even to the top of the spinal cord itself. I have seen anaesthesia of the 
conjunctiva and cornea ensue upon section of the so-called ascending, really descending, root of 
the Vth at the level of the calamus scriptorius.\ It is worth inquiring whetlier the three great 
divisions of the trigeminus springing from the Gasserian ganglion may not themselves be segment- 
ally collected portions of the nerve, each representing one or more complete cranial sensory roots. 
If so, the three great peripheral divisions will each possess a zonal skin-field extending from 
mid-dorsal line to mid-ventral line. To examine this point, the delimitation of the skin-field 
belonging to each of the three divisions of the Vth was undertaken. 
First Division of the Vth {Ramus ophthalmicus). See fig. 2 in text. 
The skin-field belonging to this division of the Vth cranial nerve lias been separately 
isolated in two individuals ; the results of the two experiments agree very closely indeed. 
Experiment. — M. rhesus. Male, young. 15.9. 1893. 
Measurements : — 
Supra-sternnl notch to pubic crest ... ... ... 25 centims. 
Acromion process to tip of middle iligit ... ... 31 „ 
At 9.20 a.m., the posterior roots of the Ilnd, Ilird, and IVth cervical nerves of the right 
side, and the inferior and middle divisions of the right Vth cranial nerve, severed after the dura 
mater had been opened. At 5.30, the isolated field supplied by the ramus ophthalmicus finally 
delimited. 
' The isolated field of response lies above and lateral to the palpebral fissure. It is bounded 
by a line which, starting from the mid-dorsal line of the head at a level of about i centim. behind 
* In this statement the tiny and segmentally incomplete field of the vagus, described p. 62, is intentionally left out of account, 
t Sherrington, ' Joiirn. of Physiol,,' vol. 14, pp. 293, etc., 1892. 
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