I230 HUMAN ANATOMY. 



the external rectus muscle and, directed medially, crosses above the levator palpebrae 

 superioris and superior rectus and reaches the superior oblique, which it enters on 

 the upper surface close to the external border (Fig. 1056). 



The communications of the trochlear nerve, as it courses in the wall of the 

 cavernous sinus are : (i) filaments from the carotid sympathetic plexus; (2) fibres 

 of common sensation from the ophthalmic division of the fifth. 



Variations. — The course of the trochlear nerve is sometimes through instead of over the 

 levator palpebrae superioris. Unusual branches to sensory nerves, as the frontal, supratroch- 

 lear, the infratrochlear and the nasal, are probably due to the aberrant course of sensory fibres 

 from the trifacial. The fourth nerve occasionally sends a branch to the orbicularis palpebrarum. 



THE TRIGEMINAL NERVE. 



The fifth, trigeminal or trifacial nerve (n. trigeminus), the largest of the cranial 

 series, is a mixed nerve and consists of a large sensory part (portio major) and a 

 much smaller motor portio7i (portio minor). The former supplies fibres of common 

 sensation to the front part of the head, the face, a portion of the external ear, the 

 eye, the nose, the palate, the naso-pharynx in part, the tonsil, the mouth and the 

 tongue. The motor portion is distributed to the muscles of mastication, the mylo- 

 hyoid and the anterior belly of the digastric. The relation of the fibres composing 

 these two parts to the cells within the brain-stem is, therefore, very different, in the 

 case of the motor fibres the cells being a nucleus of origin and in that of the sensory 

 fibres one of reception. 



The Sensory Part. — The fibres comprising the sensory part of the trigeminal 

 nerve, which convey sensory impulses from the various head-structures, are the pro- 

 cesses of cells lying outside the central axis in the Gasserian ganglion on the sensory 

 root. The portions of the fibres between the periphery and the ganglion correspond 

 to elongated dendrites, while the much shorter centrally directed constituents of the 

 sensory root, connecting the ganglion with the brain-stem, are the axones of the 

 Gasserian neurones. The general resemblance between the fifth cranial nerve and a 

 typical spinal nerve is striking, in each case the sensory root bearing a ganglion and 

 the motor root proceeding from cells within the central nervous axis. 



Proceeding brainward as axones of the Gasserian cells, the sensory fibres of the 

 trigeminal nerve become consolidated into the large sensory root, which passes 

 through an opening in the dura mater (Fig. 1033) situated beneath, the attachment 

 of the tentorium cerebelli to the posterior clinoid process. Coursing backward 

 through the posterior fossa of the cranium it enters the brain-stem on the lateral sur- 

 face of the pons, slightly behind the superior border, as the conspicuous group of 

 robust bundles that mark the superficial origin of the nerve (Fig. 1046). Just above 

 it is the superficial origin of the motor root, from which it is separated by a small 

 bundle of pontine fibres which belong to the middle cerebellar peduncle. Below and 

 in line with it are the superficial origins of the facial and auditory nerves. 



Entering the tegmental portion of the pons, close to the overlying superior cerebellar 

 peduncle, the sensory fibres soon come into relation with the extensive trigeminal reception- 

 nucleus, a columnar mass of gray matter within the lateral part of the tegmentum (Fig. 935). 

 This nucleus extends from the middle of the pons through the entire length of the medulla and 

 into the spinal cord as far down as the level of the second cervical segment, where it becomes 

 continuous with the substantia gelatinosa of the cord. The rounded and enlarged upper end 

 of this tapering column is described as the sensory nucleus of the fifth nerve, although it com- 

 prises only a small part of the reception-nucleus. The latter, in turn, is the upward prolongation 

 of the substantia gelatinosa Rolandi, conspicuous in all cross-sections of the lower pons and 

 medulla as an oval field of gray matter (Fig. 930). 



On nearing this column the sensory fibres divide into ascending and descending branches, 

 much in the same way as the posterior root-fibres bifurcate within the posterior columns of the 

 cord. The ascending fibres, distinctly finer than the descending, soon penetrate the sensory 

 nucleus and the substantia gelatinosa and end in arborizations around the neurones of the 

 reception nucleus. The coarser descending fibres become collected into a compact bundle, the 

 descending or spinal root (tractus spinalis n. trigemini), whose medially directed concavity closely 

 embraces the lateral surface of the column of gray substance. Beginning with its descent, the 



