THE EXTRAPYRAMIDAL MOTOR SYSTEM 895 



FIG. II. Neuronal mechanism for turnina; movements in the horizontal plane to the left side. 

 The apparatus for ipsiversive turning is schematically drawn on the left of the figure, that of contra- 

 versive turning on the right. From the vestibular nuclei (Neve) arises the ipsilateral vestibuloreticulo- 

 thalamic tract which is called the dorsolateral tegmental fascicle (F.t.dl) in its first part and, more 

 rostrally, Forel's tegmental fascicles (F.Fo). The fibers of this fascicle terminate in the ventrointer- 

 mediate nucleus of the thalamus (V.im). From there arise the cortical projections to the central 

 region, probably area ja. One of the contraversive turning systems starts from the anterior thalamic 

 nucleus (A.pr) and reaches area 2^ of the gyrus cinguli. This area and all adversive cortical areas 

 (5, 6, J, 22) send fibers through the internal capsule iCa.i) to the entopeduncular nucleus (Pall.i). 

 This nucleus also receives fibers from the caudate nucleus (Cd). The efferent tract runs through the 

 zona incerta iZ-i) near the subthalamic nucleus and crosses the mid-line in the anterior midbrain. 

 It is connected with the reticular formation which contains the apparatus for all turning move- 

 ments to the same side. Its efferent tract is mainly the reticulospinal tract (T.rl.fp). Another pathway 

 for contraversive movements arises in area /8 and passes through the tectum opticum (T.op) where 

 it is connected with the substrate of optic grasp reaction. The efferent path, the tectospinal tract 

 (T.t.sp) also mediates contraversive turning. It crosses the mid-line and descends as the predorsal 

 fascicle (F.pr.d). [From Hassler (89).] 



Stimulus frequencies below lo per sec. The eye move- 

 ments precede the body turning in the same direction. 

 Stimulation of many areas simultaneously produces 

 a dilatation of the pupils. Contraversive turning 

 movements easily merge into circling movements. 

 The latter have been described above as related 

 also to the caudate nucleus. The conjugate eye 

 moveinents produced by stimulation of the tectum 

 opticum, also called visual grasp reactions (Akert 

 and Hess), rarely turn into circling movements. 



Following stimulation of the cingulate gyrus the body 

 and head follow the movements of the eyes to the 

 contralateral side only after some time. In man these 

 contraversive movements are almost completely 

 restricted to the eyes. Such a deviation conjiiguee can 

 be shown experimentally in an encephale isole prepara- 

 tion in which movements of the trunk and limbs are 

 impossible [Hyde & Eliasson (124, 125)]. In un- 

 restrained cats passive holding of the head may be 

 sufficient to induce an isolated conjugate movement 



