1228 HUMAN ANATOMY. 



Intracranial Course. — Leaving their deep origin as the axones of the nuclear 

 cells, the oculomotor fibres sweep in ventrally directed curves (Fig. 963) through 

 the posterior longitudinal bundle, tegmentum, red nucleus and inner margin of the 

 substantia nigra and, collected into about a dozen root-bundles, have their super- 

 ficial origin along a shallow groove, the oculomotor sulcus (Fig. 974), on the 

 medial surface of the cerebral peduncle, just in front of the pons and at the side of 

 the interpeduncular space. 



Beyond this superficial origin, the linear group of root-fibres soon becomes 

 consolidated into the large and conspicuous trunk of the third nerve, although not 

 infrequently one root-bundle emerges more laterally from the ventral surface of the 

 cerebral peduncle and for a short distance remains separated from the other constit- 

 uents. The nerve courses forward and outward from the posterior perforated space, 

 between the posterior cerebral and superior cerebellar arteries, to the outer side 

 of the posterior cHnoid process, where, in the triangular interval between the free 

 and attached borders of the tentorium, it enters the dura (Fig. 1033). Embedded 

 within this membrane, the nerve follows the upper portion of the outer wall of the 

 cavernous sinus and leaves the cranium by entering the orbit through the sphenoidal 

 fissure. On gaining the median end of the fissure the nerve divides into a superior 

 and an iyiferior branch, which enter the orbit by passing between the two heads of 

 the external rectus muscle, in company with, but separated by, the nasal branch of 

 the trigeminal nerve, the sixth nerve lying below. 



Branches and Distribution. — The superior branch (ramus superior) (Fig. 

 1051), the smaller of the two, passes upward, over the optic nerve, to the superior 

 rectus muscle, which, together with the levator palpebrae superioris, it supplies. In 

 both cases the nerve enters the ocular surface of the muscle. 



The inferior branch (ramus inferior) (Fig. 1051) is directed forward and, 

 after giving of^ twigs to the ocular surface of the internal and inferior recti, is 

 continued below the eyeball, between the inferior and external straight muscles, to 

 supply the inferior oblique, whose posterior border it enters. This, the longest 

 branch of the oculomotor nerve, in addition to sending one or two fine twigs to the 

 inferior rectus, contributes a short thick ganglionic branch (Fig. 105 ij, which joins 

 the postero-inferior part of the ciliary ganglion (page 1236) as its short or motor 

 root and conveys fibres destined for the sphincter pupillae and ciliary muscles. 

 Sensory fibres from the ophthalmic division of the fifth nerve are distributed to the 

 muscles along with the fibres of the third, having joined the latter before it entered 

 the orbit. Similarly in the wall of the cavernous sinus, the nerve is joined by 

 sympathetic fibres from the cavernous plexus on the internal carotid artery. 



Variations. — These conijist, for the most part, of unusual branches which at times seemingly 

 replace one of the other motor orbital nerves. Thus, the third nerve may give a branch to the 

 external rectus, either in addition to, or to the exclusion of the sixth, which may be absent ; or 

 it may give a filament to the superior oblique. Minor deviations in the course of its branches, 

 such as piercing the inferior rectus or the ciliary ganglion, have also been recorded. 



THE TROCHLEAR NERVE. 



The fourth or trochlear nerve (n. trochlearis), also called the pathetic, is the 

 smallest of the cranial series and supplies the superior oblique muscle of the eyeball. 

 The deep origin of the nerve is from the trochlear micleus, a small oval collection 

 of cells situated in the ventral part of the gray matter surrounding the Sylvian aque- 

 duct, that extends from opposite the upper part of the inferior quadrigeminal body 

 to the lower pole of the superior colliculus. This nucleus, about 2 mm. in length, 

 lies near the mid-line and immediately below (caudal to) that of the third nerve, 

 from which, however, it is distinct, being separated by a narrow interval from the 

 ventral part of the oculomotor nucleus. It lies in intimate relation with the pos- 

 terior longitudinal fasciculus in a distinct depression on the dorsal surface of that 

 bundle (Fig. 960). In structure the trochlear nucleus resembles that of the oculo- 

 motor, its nerve-cells including those of large, medium and small size. 



Arising from the nucleus, the root-fibres of the fourth nerve pursue a course 

 of considerable length within the mid-brain before gaining their superficial origin. 



