Before attempting to decide how far Marshall’s views about the so-called anterior roots of the seventh, the fifth and the third nerves are well founded it will conduce to clearness to state the characters and relations of the two roots of spinal nerves.
The posterior root is (1) always purely sensory; (2) it always develops a ganglion. The anterior root is (1) always purely motor; (2) it always joins the posterior root below the ganglion, except in Petromyzon (though not in Myxine) where the two roots are stated to be independent.
How far do Marshall’s anterior and posterior roots of the cranial nerves exhibit these respective peculiarities?
With reference to the sixth and seventh nerves he states “we must regard the sixth nerve as having the same relation to the seventh that the anterior root of a spinal nerve has to the posterior root.” On this I would remark (1) that the posterior root of this nerve is a mixed sensory and motor nerve and therefore differs in a very fundamental point from that of a spinal nerve; (2) the sixth nerve though resembling the anterior root of a spinal nerve in being motor and without a ganglion, differs from the nearly universal arrangement of spinal nerves in not uniting with the seventh.
With reference to the fifth nerve it is to be observed that it is by no means certain that the whole of the motor fibres are supplied by the so-called anterior roots, and that these roots differ again in the most marked manner from the anterior roots of spinal nerves in joining the main root of the nerve above (nearer the brain), and not as in a spinal nerve below the ganglion. The gangliated root of the third nerve is purely motor[180], and its so-called anterior roots again differ from the anterior roots of spinal nerves, in the same manner as those of the fifth nerve.
With reference to the glossopharyngeal and vagus nerves I would merely remark that no anterior root has even been suggested for the glossopharyngeal nerve and that the posterior roots of both these nerves contain a mixture of sensory and motor fibres.
In view of these facts, my original hypothesis appears to me to be confirmed by Marshall’s observations.
The fact of all the posterior roots of the above cranial nerves (except the third which may be purely motor) being mixed motor and sensory roots appears to me to demonstrate that the starting-point of their differentiation was a mixed nerve with a single dorsal root; and that they did not therefore become differentiated from nerves built on the same type as the spinal nerves with dorsal sensory and ventral motor roots. The presence of such non-gangliated roots as those of the third and fifth nerves is not a difficulty to this view. Considering that the cranial nerves are more highly differentiated than the spinal nerves, and have more complicated functions to perform, it would be surprising if there had not been developed nonganglionated roots analogous to, but not of course homologous with, the anterior roots of the spinal nerves[181].
As to the sixth nerve further embryological investigations are requisite before its true position in the series can be determined; but it appears to me very probable that it is a product of the differentiation of the seventh nerve.
The fourth nerve. No embryological investigations have been made with reference to the fourth nerve. It is possible that it is a segmental nerve comparable with the third nerve, and that the only remnant still left of the segment to which it belongs is the superior oblique muscle of the eye. If this is the case there must have been two præmandibular segments, viz. that belonging to the third nerve, and that belonging to the fourth nerve. Against this view of the fourth nerve is the fact, urged with great force by Marshall, that the superior oblique muscle is in front of the other eye muscles, and that the fourth nerve therefore crosses the third nerve to reach its destination.