THE MESENCEPHALON. 



II 17 



the vestibular (Deiters') 

 element of the posterior 

 establish reflex paths for 

 fibres, both crossed and 



Notwithstanding the admitted importance of the tract and the prolonged study that it has 

 received, much remains to be determined concerning the source and connections of the many- 

 constituents which undoubtedly go to form the bundle. Among the more certain of these com- 

 ponents the following may be mentioned : 



1. At the upper end of the fasciculus a considerable number of fibres arise from the cells 

 of the nucleus of the posterior commissure, or Darkschewitscli' s nucleus, which lies in advance 

 of the oculomotor nucleus, within the gray matter surrounding the superior end of the Sylvian 

 aqueduct. According to Edinger an additional contingent takes origin from a nucleus (n. fas- 

 ciculi longitudinalis medialis) within the gray matter of the 



floor of the third ventricle in the vicinity of the corpus mam- 

 millare. The contributions from both these sources join 

 the fasciculus as crossed fibres from the nuclei of the 

 opposite side. 



2. The fibres arising from 

 nucleus constitute an important 

 longitudinal bundle, since they 

 equilibration impulses. These 



uncrossed, join the fasciculus and pass in both directions. 

 Those passing brainward have as their chief objective point 

 the oculomotor nucleus, although the nuclei of the sixth 

 and fourth nerves receive fibres or collaterals. In this 

 manner the filaments supplying the various ocular muscles 

 are brought under the influence of the vestibular impulses. 

 It is probable that the facial nucleus likewise receives 

 collaterals, if not main stems, of the vestibulo-nuclear fibres. 



3. Upon clinical and experimental evidence, it may be 

 assumed that fibres pass by way of the longitudinal bundle 

 from the abducent nucleus to that part of the oculomotor 

 nucleus sending fibres to the internal rectus muscle of the 

 opposite side (perhaps also from the nucleus of the third 

 nerve to that of the abducens of the same side), by which 

 arrangement the harmonious action of the internal and 

 external recti muscles is insured. Basing their conclusions 

 upon similar evidence, many anatomists accept the 

 existence of fibres which pass by way of the posterior 

 longitudinal bundle from the oculomotor nucleus to the cells 

 of the facial nucleus (page 1251) from which proceed the 



fibres supplying the orbicularis palpebrarum and the corrugator supercilii. In this manner 

 the coordinated action of these muscles and the levator palpebrae superioris is explained. A 

 similar connection is probably established by the posterior longitudinal bundle between the 

 nucleus of the hypoglossal and that of the facial nerve, whereby the closely associated 

 movements of the lips and tongue are assured. That the function of the posterior fasciculus 

 is by no means limited to association of the nuclei of the ocular nerves is evident from the 

 fact that in animals or individuals in which such centres are wanting (due to absence or 

 imperfect development of the visual organs ) the bundle is nevertheless well represented. 



4. Fibres arise from the reception-nuclei of the remaining sensory nerves of the brain-stem 

 and pass to the posterior longitudinal fasciculus of the same and the opposite side. On enter- 

 ing the bundle, they course in both directions and by means of their collaterals and stem-fibres 

 send end-brushes to the nuclei of the motor nerves, in this manner establishing direct reflex 

 areas between the afferent and eflferent paths. 



Strictly considered, it is probable that the fibres establishing connections with the nuclei of the 

 sensory nerves constitute a small separate tract, lying within the central gray matter dorso-lateral 

 to the posterior longitudinal bundle. This path has been called the /asciculus longitudinalis 

 dorsalis of Schiitz, while the main bundle is then termed the fasciculus longitudinalis medialis. 

 In order to avoid confusion, both sets of fibres are here regarded as parts of one path, the pos- 

 terior longitudinal bundle. 



DEVELOPMENT OF THE MESENCEPHALON. 



Diagram showing chief constituents 

 of posterior longitudinal fasciculus. Ill, 

 IV, VI, VII, XII, nuclei of respective 

 nerves; D, vestibular I, Deiters') nucleus; 

 CN, common nucleus of posterior 

 commissure and posterior longitudinal 

 fasciculus. 



Of the three primary cerebral vesicles, the mid-brain undergoes least change. Although 

 much smaller than either of the other segments of the brain-tube, its prominent position, lying 

 as it does at the summit of the cephalic flexure, makes it conspicuous in the early developing 

 brain. During the enormous expansion upward and backward incident to the development 

 of the cerebral hemispheres in man, the mid-brain becomes covered in and deposed to a 

 dependent position and a relatively small size. For a time possessing a spacious cavity, it fails 

 to keep pace with the growth of the adjoining parts ; its walls thicken and its lumen becomes 

 eventually reduced to the narrow Sylvian aqueduct. 



