THE MAMMALIAN NERVOUS SYSTEM 77 



92. The longitudinal medial bundle. — The fasciculus longitud- 

 inalis medialis (the posterior longitudinal bundle) is an important 

 longitudinal motor co-ordination system which can easily be 

 followed in the microscopic sections through the whole length 

 of the midbrain and bulb and downward into the fasciculus pro- 

 prius of the cord. It is composed of heavily myelinated fibers 

 lying near the median plane immediately ventrally of the 

 ventricle. It has already been noted in connection with the 

 vestibular apparatus (Section 83). If its entire course has not 

 already been entered in the drawings, this should be done now. 

 It arises in front of the III nucleus and is related with all of the 

 motor nuclei of the brain and spinal cord. It is used in conju- 

 gate movements of the eyeballs, in vestibulo-oculomotor equil- 

 ibratory reactions, and in many other reflex movements. See 

 Barker ('01), Fig. 406, p. 617; Bruce ('92), Plate XXVII, Fig. 1; 

 Cunningham ('15), Fig. 524, p. 590; Piersol ('13), Fig. 965, p. 

 1117. 



93. Dissection of the longitudinal medial bundle. — On the right 

 side of the divided sheep's brain this tract can be brought to 

 view by gentle scraping in the longitudinal direction on the 

 median cut surface immediately ventrally of the fourth ventricle. 

 In this way the fasciculus can be exposed throughout the entire 

 length of the brain stem. 



94. Microscopic study of the pyramidal tract. — The tractus 

 cortico-spinalis (fasciculus cerebro -spinalis of the B N A), or 

 pyramidal tract, is the great descending voluntary motor path 

 between the motor areas of the cerebral cortex and the lower 

 motor centers. It can be readily identified in the sections 

 through the middle regions of the medulla oblongata (see refer- 

 ences in Section 62), lying ventrally close to the median plane 

 below the olives. From the level of the olives follow it upward 

 also. Immediately below the pons it forms a sharp projection 

 on the ventral surface, the pyramid, from which the tract 

 receives its name. Its fibers can be followed into the substance 

 of the pons. Here, as the sections are followed upward, the fibers 

 of the cortico-pontile tracts are added to them, so when they 

 emerge upon the ventral surface of the cerebral peduncle above 

 the pons, the pyramidal fibers form only the middle part of the 

 basis pedunculi, with cortico-pontile fibers on each side of them. 



