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MANUAL OF HISTOLOGY. 



FIG. 144. Diagram showing ori- 

 gin of the third and fourth nerves 

 from the gray matter about the aque- 

 duct of Sylvius: c. a, cms cerebri; 

 8, third nerve ; 4, fourth nerve. 



sensory root, and Meynert makes it one of its points of 

 origin. 



The sensory root is reinforced by fibres from a group of 

 large oval cells external to the fourth ventricle and by the so- 

 called descending branch (Meynert), which is seen in trans- 

 verse section in the same location com- 

 ing from regions still higher up. Some 

 fibres also come from the raphe and 

 arcuate fibres, and others from the low- 

 er sensory origin of the fifth, which 

 occupies a lateral position in all the 

 sections up from the spinal accessory 

 region of the medulla. 



Higher in the pons, where the ante- 

 rior motor tracts or pyramids, before 

 mentioned, begin to separate into the 

 crura cerebri, the fourth nerves are 

 seen. They are supposed to arise from 

 a nucleus at the floor of the fourth ven- 

 tricle lower down, curve around the outer wall of the ventricle, 

 decussate in the median line in the valve of Vieussens, and 

 pass from the pons behind the tubercula quadrigemina. From 

 this point they curve forward around the crura, on the outer 

 side of which they appear at the base of the brain. 



At about this point and a little higher are seen bundles of 

 fibres emerging from the gray matter containing small cells, 

 in front of the fourth ventricle, diverging and pursuing an 

 arcuate course through the crura, to converge again and emerge 

 from the inner side of each crus. (See Fig. 144.) This consti- 

 tutes the nucleus of origin, the course and point of emergence 

 of the third nerve a view hard to get unless just the right 

 obliquity is given to the section. 



Imbedded in the crus, in the region through which the third 

 nerve passes, is a collection of pigmented cells forming the locus 

 niger. Higher the crura separate and enter their respective 

 hemispheres. Their further course is better shown by a trans- 

 verse vertical section of the hemispheres at the large part of 

 the thalamus opticus. (See Fig. 145.) 



Here we see a great part of the substance of the crus flat- 

 tened in form passing upward, between the optic thalamus and 

 a gray mass called the nucleus lenticularis, forming what is 



