832 THE NERVOUS SYSTEM 



(2) Certain of the descending cerebro-pontile fibres terminate about cells of the nuclei of 

 the pons. Such cells give off fibres which probably, for the most part, pass to the cerebellar 

 hemispheres, the impulses from the cerebral hemisphere of one side being conveyed to the 

 opposite cerebellar hemisphere. Most of the descending cerebro-pontile fibres are thought to 

 cross the mid-line to terminate about cells of the nuclei of the pons of the opposite side, a rela- 

 tion not sufficiently emphasised in the accompanying diagram (fig. 657). 



Of the cerebro-pontile paths, the frontal pontile path (Arnold's bundle) is described as 

 arising in the cortex of the frontal lobe (frontal operculum) passing in the anterior portion of 

 the internal capsule down into the medial part of the base of the cerebral peduncle, and terminat- 

 ing in the grey substance of the pons. The descending temporal pontile path, sometimes 

 called Turk's bundle, arises in the cortex of the temporal lobe, traverses the posterior portion 

 of the internal capsule, lies lateral in the pyramidal portion of the cerebral peduncle, and termi- 

 nates in the grey substance of the pons. In the posterior part of the internal capsule, the tem- 

 poral pontile path is joined by a small bundle arising in the occipital lobe and going to the 

 pons nuclei. This, supposedly smaller than the other two, adds an occipito-pontile path. 



The total area in cross section of the pyramidal fasciculi as they enter the pons above is 

 considerably greater than that which they possess as they emerge as the pyramids of the medulla 

 below. The difference is considered very appreciably greater than can be explained as due 

 to the loss of pjTamidal fibres supplied to the nuclei of origin of the cranial nerves lying within 

 the level of the pons, and the additional difference is explained as due to the termination 

 within the pons of the cerebro-pontile paths. 



THE ISTHMUS OF THE RHOMBENCEPHALON 



The isthmus of the rhombencephalon is nothing more than the transition of 

 the metencephalon into the mesencephalon above. It is quite short and com- 

 prised of only the structures which run through it, namely, the hrachia conjunc- 

 tiva (superior peduncles of the cerebellum), the anterior medullary velum, the 

 lateral sulcus of the mesencephalon, the cerebral peduncles, and the inferior end of 

 the interpeduncular fossa. It surrounds the superior extremity of the fourth 

 ventricle. The lateral and medial lemnisci, the superior extension of the nucleus 

 of the trigeminus, the mesencephalic nucleus and root of the masticator nerve and 

 Gowers' tract extend through it. At the mid-line, just inferior to the inferior 

 quadrigeminate bodies is the frenulum of the anterior medullary velum and the 

 trochlear nerves, emerging at the sides of this, course ventrally around the sides 

 of the isthmus. In the lateral sulcus, the isthmus shows usually a small triangular 

 elevation known as the trigonum lemnisci from the fact that the lateral lemniscus 

 tends toward the surface in this region. 



Functions of the cerebellum. — From the above descriptions involving the structures of the 

 metencephalon, it may be noted (1) that a given side of the cerebellum is associated chiefly 

 with the same side of the general body and with the opposite side of the cerebrum. (2) That 

 it receives afferent impulses from the spinal cord (brought into the cord by the dorsal roots of 

 the spinal nerves) by way of the direct cerebellar fasciculus of the same side, and bj^ Gowers' 

 tract and from the nuclei of the fasciculus gracilis and cuneatus of the same and opposite sides. 

 It further receives afferent impulses from the nuclei of termination of the trigeminus, glosso- 

 pharyngeal and vagus of the same side chiefly, and especially does it receive afferent impulses 

 from the nuclei of the vestibular nerve of the opposite and same side. (3) That the cerebellum 

 sends impulses to the red nucleus, the thalamus and the cerebral cortex of the opposite side, 

 and some of its fibres terminate in the nuclei of termination of the vestibular nerve and probably 

 some fibres arising in its roof nuclei descend into the spinal cord direct. (4) That the cerebel- 

 lum receives impulses from the thalamus of the opposite side by way of the thalamo-olivary 

 tract and the inferior olive, and especially from the cerebral cortex of the opposite side by way 

 of the frontal, temporal and occipital pontile paths and the nuclei of the pons. Further, 

 fibres from the general pyramidal fasciculi are described as terminating about cells of the nuclei 

 of the pons. 



Taking into consideration these known associations of the cerebellum, the anatomically 

 possible patlis which in part may distribute cerebellar impulses to the grey substance sending 

 efferent fibres to the peripheral tissues are (1) the general pyramidal fasciculi whose cortex 

 of origin may receive impulses by fihrce proprioe from the cortical areas receiving impulses from 

 the cerebellum. The pyramidal fasciculi, decussating, distribute impulses to the grey substance 

 of the medulla and cord of tlie same side as that from wliich the ccrebello-cerebral impulses 

 passed to the cortex. (2) The lateral vestibulo-si)inal and the anterior marginal fasciculi to 

 the ventral horn of the spinal cord of the same side, prol)ably carrying impulses descending 

 from the cerebellum as well as impulses brought in by tiie vestibular nerve and descending 

 direct from its nuclei of termination into tlie spinal cord. (3) 'I'ho rubr()-s]unal tract of the 

 cord and 7)robably some of th(^ thalarno-spinal fibres (corpora-quadrigemina-thalamus path), 

 the red nuclei and thalami l)eing associated abundantly with the cerebellum. These tracts 

 likewise decussate in descending but likewise do the cerebellar impulses ascending to their cells 

 of origin. 



Whatever other functions it may possesSj developmental defects and pathologic lesions 

 show that the cerebellum has to do with the equilibration of the body and the finer coordinations, 

 adjustive control of tlie contractions of functionally correlated groups of muscles. Making this 



