318 
BULLETIN OP THE UNITED STATES FISH COMMISSION. 
The relations of the sensory nerves in the head of Menidia are exhibited in the 
preceding diagram. The somatic sensory or general cutaneous nerves of the head all 
terminate in the sp. V tract, which is the continuation into the head of the dorsal 
horn of the spinal cord and which is represented in human anatomy by the “ascend- 
ing root” and the chief sensory nucleus of the fifth nerve. This system receives a 
small root from the vagus nerve and all of its other fibers from the trigeminus, com- 
prising the whole of the sensory i^ortion of the latter nerve. The general cutaneous 
component appears at first sight to have been reduced in the head as compared with 
the trunk. Such, however, is not the case; it has merely been compacted. On the 
other hand, the skin of the face is certainly more richly supplied with general cutaneous 
(tactile) nerves than areas of corresponding size on the trunk. 
The communis system of the head corresponds, in my judgment, with the viscero- 
sensory system of the trunk. Under this term I include all nerves which supply the 
mucous lining of the mouth and pharynx, sensory fibers of the r. visceralis vagi, and 
those for taste buds and their allies, the terminal buds of the outer skin. In Menidia 
all of these nerve fibers terminate in a single center, the lobus vagi, which corresponds 
to the sensory nucleus of tlie vagus in man. In some other bony fishes there is, in 
addition, a similar pre-auditory center, the so-called lobns trigemini (which is not, 
however, homologous with the “lobus trigemini” of ganoids and selachians). These 
communis fibers enter by way of the X, IX, and YU nerves, and are not represented 
in any other cranial nerve roots. Like the general cutaneous nerves, they have been 
unified into a very compact system with a single center. This system has been 
enormously hypertrophied in the head, and for a double pur[»ose. In the first place, 
the viscero-seusory nerves of the trunk seem to have been in large measure supplanted 
by the r. intestinalis vagi. In the second place, the cephalic end of the digestive tract 
requires a proportionally greater nerve supply for its elaborate branchial and labial 
apparatus. And, in connection with the latter, more highly specialized sense organs 
(taste buds) have been developed in response to an obvious functional need. The 
advantage to be derived from such a centralization of the sensory apparatus of the 
entire digestive tract is obvious. In Menidia the fibers of this system enter the lobus 
vagi either directly by the vagus or through the mediation of the fasciculus communis 
from the IX and VII nerves. 
The acu Stic o -lateral system has no direct representative in any spinal nerve. To 
it belong all the nerves which suq)ply the lateral line organs and the organs of the 
internal ear. These nerves all terminate together in the tuberculum acusticum, and 
the auditory nerve is the only survivor of this system in the higher vertebrates. 
What may be the relationshii) between this system and the other, probably phyloge- 
netically older, sensory systems is as yet problematical; it has probably been derived 
from the general cutaneous. 
It is obvious that i.i making comparisons between cranial and spinal nerves more 
attention should be paid to the morphological dilferences between these several types 
of nerve fibers than has hitherto been done by most morphologists and embryologists. 
There is to-day as little justification for the direct homologizing of general cutaneous 
nerves of the trunk with, say, the nerves supplying laste buds in the head as there 
would be for the comparison of a n\otor with a sensory root; and yet it is not infrequent 
to see dorsal spinal roots compared with all dorsal cranial roots indiscriminately. 
