Nerves, Spinal Coed, Brain and Other Ganglia 89 



Fig. 72. Diagram showing the course, origin, and termination of the fibers of the 

 principal tracts of the white matter of the spinal cord. (Quain's Anatomy.) ' 



Descending tracts : 



ia, a fiber of the crossed pyramidal (crossed cortico-spinal ; crossed lateral pyramid) 

 tract. lb, An uncrossed fiber of the pyramidal tract, descending in the lateral column 

 of the same side. 2, a fiber of the direct pyramidal (direct cortico-spinal ; uncrossed 

 pyramidal; ventral pyramidal) tract, or "bundle of Tiirck." 3, a fiber of the antero- 

 lateral (ventro-lateral) descending (ponto-spinal) tract or " anterior marginal bundle 

 of Lowenthal ". 4, a fiber of the rubrospinal (prepyramidal) tract or "bundle of 

 Monakoff ". 5, a fiber of the comma tract. 



Ascending tracts : 



6, a fiber of the postero-mesial (dorso-mesial) spino-bulbar tract, or " column of 

 Goll ". 7, Fibers of the postero-lateral (dorso-lateral) spino-bulbar tract, or " column 

 of Burdach ". 9, a fiber of the direct cerebellar (posterior spino-cerebellar) tract or 

 " Flechsig's tract ". 10, a fiber of the antero-lateral ascending (anterior spinal cere- 

 bellar) tract, or " Gower's tract ". 



The optic nerve differs in an important way from the other cranial 

 nerves. These latter are analogous to spinal nerves, being composed of 

 fibers growing out of the central nerve tube, or from ganglia correspond- 

 ing to the spinal root ganglia; but the optic nerve and the inner layers 

 of the retina are analogous to lobes of the brain (to the olfactory tract, for 

 instance) , since they contain, not fibers of peripheral afferent neurons, but 

 fibers of intermediate or associative neurons. [Fig. 61.] 



III. The oculo-motor nerve (mixed) arises from an extensive 

 nucleus lying along the front and central portion of the brain stem just 

 above the pons (in the floor of the aqueduct of Sylvius and the third 

 ventricle), emerging from the inner margin of the crusta. The cells in 

 the anterior part of this nucleus send axons to the ciliary ganglion, in which 

 they connect with neurons running to the intrinsic muscles of the eye, 

 viz., the ciliary muscle and the sphincter pupillae (sphincter iridis). The 

 cells of the remainder of the nucleus send axons to certain of the 

 extrinsic muscles of the eye, viz., to the recti, (excepting the external 

 rectus), the inferior oblique, and the levator palpebrarum. Along with 

 these axons, dendrites from afferent cells in the nucleus are sent to the 

 same muscles. 



IV. The trochlear or pathetic nerve (mixed) arises from a nucleus 

 behind that of the third nerve, in the floor of the aqueduct of Sylvius at 

 the level of the inferior corpora quadrigemina. The fibers emerge through 

 the valve of Vieussens (the thin plate which forms part of the anterior 

 wall of the fourth ventricle in front of the cerebellum). The fibers of 

 this nerve supply the superior oblique muscle of the eye. 



V. The trigeminal (or trifacial) nerve (mixed) has three branches: 



