Page:The New International Encyclopædia 1st ed. v. 14.djvu/426

* NERVOUS SYSTEM AND BRAIN. 376 NERVOUS SYSTEM AND BRAIN. cord occurs llie iiyraiiiidal docussaliun. Above this decussation the fibres of the main cortico- spinal motor tracts lie in two large bundles, one on cither side of the anterior median fissure, and known as the anterior pyramids. In the decussa- tion of the pyramids the majority of these fibres cross over to the opposite posterolateral region and pass down into the cord as the crossed pyramidal tract, the few fibres remaining next Vayruui^ Zing ^I^fpaner, 6ECT10S OP Tlip MEUCLLA OBLOS0A1 A AT ABOTT THE SlIUDLE OK THE OLIVAKY BODY. Sue. fun. /tT., nucleus of ftinicuhis teres; Vag. tiuc/., vagus nuclei; Lint/., lingulu; Xur. fira., nucleus gracilis; JV'«c. cu;i., nucIeUB cuncatus; Corp. rest., corpus reslifornie ; Asc. rt. .''th Mcr., .'uiccitdiiig root of 5th nerve; Vait. rt.^ vagus root; .4rci. jih.^ arciform fibres ; Otiv. jjmc, olivary nucleus; lltipo, n'er., hypoglossal nerve; Ant. nwtl. jUs.^ anterior niediati fissure ; Iltjpo. nuc, liypoglossal nucleus ; liap.y median raphe ; Form, recti, formatio reticularis ; font. ant. co/., continuation of the anterior column ; Access, oliv. nuct.^ accessory olivary nuclei. the anterior fissure forming the direct pyramidal tract. The crossing fibres of the decussation sep- arate off tlio anterior horns from the rest of the gray matter, while the intermingling of white matter and gray matter is known as the formr.tio reticularis. Shortly above the pyramidal decus- sation masses of gray matter l)cgin to appear in the posterior columns. These are known respect- ively as the nucleus gracilis or nticlcus of the column of Goll and the nucleus cuneatus or nucleus of the column of Burdach. In these nuclei terminate most or all of the fibres of the columns of (!oll and of Burdach. The nuclei increase in size pari passu with the decrease in the size of the columns, the latter finally entirely disappearing. These nuclei are therefore nuclei of termination for the fibres of these columns and arc of the nature of a relay station in the main spino-cortical sensory conduction path. From cells in these nuclei, axones pass around the cen- tral canal — internal arcuate fibres — decussate in the median line — sensory decussation — and then turn upward as the fillet or lemniscus. This is the main continuation brainward of the great sensory tract. Some axones from cells in the nuclei do not turn upward in the fillet, but. skirt- ing the ventral stirface of the pyramids, as the external arcuate fibres, pass to the cerebellum in the rcstiform body. Of the lateral tracts of the cord, the direct cerebellar tract joins the rcsti- form body and thus reaches the cerebellum through its inferior pedtmclc. while the tract of Oowers continues upward in the hiteral region of the medulla, its exact terminations being as yet tindeterniincd. The posterior horns become, in the medulla, separated from the rest of the pray matter, and, becoming broken up, serve as nuclei of termination for the sensory divisions uf some of the cranial nerves. As it passes up through the medulla, the central canal, surrounded by an increased amoimt of gelatinous substance, gradu- ally approaches the posterior surface, to o])en at about the middle of the medulla into the fourth ventricle. Of the new masses of gray matter which appear in the medulla, the largest are the olivary nuclei. These are irregular convoluted masses of gray matter situated in the ventral part of the medulla Ixdiind and to the outer side of the pj'ramids. They extend from the upper limit of the sensory decussation to the pons. Each olivary nucleus has a central core of white fibres which cross the median line and pass to the opposite rcstiform body forming the cerebello- olivary tract. To the inner side of the lower part of the olivary nucleus is a smaller mass of gray matter know-n as the accessory olivary nucleus, while above the main mass of the olive is the superior olivary nucleus. The distinct line of decussating fibres which extends through the centre of medulla and pons is called the median raphe. The nuclei of certain of the cranial nerves form masses of gray matter in the me- dulla. These, together with the root fibres of the nerves, will be found described below in connec- tion with the cranial nerves. The pons Varolii, or bridge, lies just above the medulla. Superficially its ventral surface is seen to be cimiposcd of fibres which ]]ass trans- versely, forming a sort of bridge between the two cerebellar hemispheres. Internally the pons is composed of tracts of fibres (white matter) and of nuclei (masses of gray matter). The fibre tracts pass both longitudinally and transversely. The longitudinal fibres are mainly the continua- tion upward of tracts already mentioned in the description of the medulla, the pyramidal tract or main motor tract, and the fillet or main sensory tract. Some of the longitudinal fibres are. however, fibres connecting the nuclei pontis w ith higher centresand commissural fibres between the nuclei pontis. The transverse fibres of the pons arc mostly fibres connecting the cerebellum with the nuclei pontis. and are consequently often called the middle cerebellar peduncles. They are composed of two sets of fibres, which from their location are known respectively as the superficial transverse pontile fibres and the deep transverse pontile fibres. In the median line the fibres decussate, forming a median raphe con- tinuous Avith that of the luedulla. ribrcs of the roots of the fifth and some of the higher fibres of the roots of the sixth and seventh nerves pass from their nuclei of origin to the ventrolateral surface of the pons. The gray matter of the pons consists of the nuclei of the nerves just mentioned and of nuclei scattered throughout the pons, which are known as the nuclei pontis and are connected with the middle cerebellar pe- duncles and with the cerebrum. Near the pos- terior surface of the pons is the fourth ventricle, which becomes narrow in the upper part of the pon' as it approaches the aqueduct of Sylvius. In the floor of the ventricle is a group of nerve cells which are so deeply pigmented as to present a dark appearance to the naked eye. This dark area is called (he locus Cicrulcus. .M)Ove the pons is the midbrain. Here the long fibre tracts are cnllected into two main bimdles known as the cerebral peduncles or crura cerebri. Each peduncle, or crus, consists of a ventral por-