FOURTH PAIR OF NERVES (nerves trochlearis, s. patheticus).—The fourth pair is the most slender of the encephalic nerves. They are intermediate in the order of succession to the third or motor oculi and the fifth nerves, and hence the name. Each nerve is attached at its encephalic extremity to the lateral part of the superior surface of the anterior medullary velum or valve of V ieussens, immediately behind the testes or the posterior of the tuhercula quadrigemina. It is divided at its attachment, for the most part, into two roots, inserted at a little distance from each other, one close to the testes, the other posterior to it. Occasionally it has but one root and sometimes even three. Gall and Spurzheim* are of opinion that the nerve might be traced to a more remote point, and according to Mayot " its fibrils appear to pass through the filaments of the pillar of the valve, and in part to arise from the back part of the medulla oblongata." The nerve is concealed at its insertion by the superior vermiform process of the cerebellum, and it is not immediately provided with neu rilemma, and hence, as also because of its delicacy, it is very easily broken off.
Its course within the cranium is circuitous and long, longer than that of the other nerves. It passes outward, downward, and forward : it first descends external to the superior peduncle of the cerebellum (the processus a cerebello ad testes), between it and the vermiform process, then becomes invested with arachnoid mem brane and free, and runs round the lateral part of the crus cerebri, above the superior margin of the pons Varolii, and beneath the free edge of the tentorium cerebelli, until it reaches the posterior clinoid process of the sphenoid bone; it then enters the outer wall of the cavernous sinus between the points of attachment of the tentorium, and is transmitted through a canal in the dura mater, by which the wall is formed, forward to the foramen lacerum of the orbit. It does not enter the sinus, being contained in a canal in its outer wall.
At the posterior part of the sinus the nerve is situate beneath the third, between it and the first division of the fifth nerve; but at the anterior, and as they are about to pass into the orbit, the fourth and frontal branch of the fifth are both above the third, the fourth internal and a little superior to the frontal.
The nerve lastly enters the orbit through the superior part of the foramen lacerum in corn= pally with the frontal, above the levator palpehrw muscle, and immediately beneath the roof of the region. having entered, it runs forward and inward, gains the surface of the superior oblique muscle, and attaching itself to it upon its superior aspect, about its middle, it divides into filaments, which are all distributed to the muscle.
The fourth nerve does not give off any branch during its course to the oblique muscle, unless, at times, first a filamentdescribed by Crtiveilhier, and, accord ing to hi m,d istributcd to the tentorium cerebelli. This filament arises from the nerve traversing the wall of the cavernous sinus, runs backward into the substance of the ten torium, and divides into two or three branches: Cruveilhier calls it " the branch of the tento rium." Secondly, according to both Swan and Cruveilhicr, the fourth nerve gives off a fila meat to the lachrymal branch of the fifth. Before its entrance into the orbit the nerve receives a filament from the sympathetic,• and at or immediately after entering, It receives one also from the frontal branch of the fifth, by the accession of which it is sensibly increased in size. It is very closely connected to the frontal itself at the back of the orbit.
Upon the fourth nerve the power of the superior oblique muscle is considered to depend. It is remarkable that this muscle should be provided with an especial nerve, differing, apparently, so much in its encephalic relations from those by which the others are supplied ; but the theories which have been advanced upon the subject are as yet so unsubstantial, that we think it better to leave them untouched. (See ORBIT, MUSCLES OF Tits).
The nerve exists with similar relations in all the vertebrata.