658 



NERVOUS SYSTEM OF THE HORSE 



the fourth ventricle. The dorsal horns of gray matter become wide, spread apart, and are broken 

 up to a great extent in the formatio reticularis. Two elongated masses of cells appear above 

 the central gray matter; these are the nucleus gracilis and nucleus cuneatus, and in them the 

 fasciculi of like name gradually end. In front of these are the terminal nuclei of the afferent or 

 sensory cranial nerves and of the sensory portions of the mixed nerves. The ventral horns are 

 succeeded by the nuclei of origin of tlie efferent or motor cranial nerves and the motor root-fibers 

 of the mixed nerves. Of the twelve pairs of cranitd nerves, the last eight are connected with 

 nuclei in the medulla and pons. The posterior olivary nucleus (Nucleus olivaris caudalis) is a 

 conspicuous gray mass which lies aljove the pyramid on each side. On cross-sections it appears 

 as a wavy layer of gray matter which is folded on itself so as to inclose a mass of white matter. 

 It is about 2 cm. long in the horse, but is smaller in circumference than in man and hence does 

 not cause any very distinct external enlargement (olivary eminence), as in the latter. It is con- 

 nected with the opposite side of the cerebellum by the cerebello-olivary fibers. At its inner side 

 are two small accessory olivary nuclei. The pyramidal tracts, which are small in ungulates, send 

 most of their fibers across to the opposite side in tlie ))osterior part of the medulla, forming the 

 pyramidal decussation. Some fibers, however, continue in the ventral column of the same side 

 of the cord, and others are connected with the nuclei of the motor nerve-roots. From the nu- 

 cleus gracilis and nucleus cuneatus fibers arise which are traceable forward to the thalamus. 



verse fissure 

 Vermis 



Hemisphere 



1 of 

 I cere- 

 \ bcl- 

 J luni 



Medulla 

 oblongata 



Olfactory 

 peduncle 



Middle 



cerebral 



artery 



Internal 



carotid 



artery 



Basilar 

 artery 



Fig. 501. — Brain of Horse, Left Lateral View. 

 The larger arteries are .shown. The nerve-roots are indicated by Roman numerals. P.c, Chorioid plexus of 

 fourth ventricle; B.p., middle peduncle of cerebellum; P, pons; H, pituitary body; L.p., pyriform lobe; A. p., 

 trigonum olfactorium; B.o, olfactory bulb; S.r.a., sulcus rhinalis anterior; S.r.p., sulcus rhinalis posterior; F.L, 

 lateral fissure (of Sylvius); F./.a., anterior branch of lateral fissure; .S. p. , presylvian fissure; S.p.n., anterior presyl- 

 vian fissure; S.s., suprasylvian fissure; ^.e., ectomarginal fissure. 



These are the internal arcuate fibers (Fibra> arcuate interntr), which curve across the median 

 plane below the central gray matter and form with those of the opposite side the decussation of 

 the fillet (Decussatio lemniscorum). Beyond the decussation the fibers form an important 

 longitudinal tract which extends forward in the mid-brain. This is the fillet or lemniscus, the 

 chief coiitiiiuation of tlie sensory conducting path from the dorsal roots of the spinal nerves. 

 The external arcuate fibers, some of which were seen on the lateral aspect of the medulla, have a 

 similar origin. Part of them (Fibra* dorsales) pass directly to the dorsal aspect of the restiform 

 body of the same side; others (Fibra» ventrales) cross to the opposite side, descend close to the 

 ventral fissure, and then curve upward and forward to the restiform body. The decussation of 

 the arcuate fibers forms the distinct median raphe seen on cross-sections of the medulla anterior 

 to the pyramidal decussation. The restiform body, situated dorso-laterally, contains, in addi- 

 tion to the arcuate fibers, the cerebello-olivary fasciculus before mentioned, and the cerebello- 

 spinal fasciculus or direct cerebellar tract. The dorsal longitudinal fasciculus corresponds to 

 the ventral ground-l)un(ne of the spinal cord, displaced dorsally by the decussation of the pyra- 

 mids and fillet. In the posterior part of the medulla it is not marked off from the fillet, along 

 the dorsal edge of which it lies. From the le^•el of the hypoglossal nucleus forward it is distinct 

 and can be traced as a conspicuous tract in the ventral margin of the gray matter of the floor of 

 the fourth ventricle and of the central gray matter of the mid-brain. Below the restiform l^ody 

 and related externally to the external arcuate fil)ers, there is a considerable bundle of longitud- 

 inal fil)ers, the spinal root (Tractus spinalis) of the fifth nerve; internal to it is the terminal nu- 

 cleus of the sensory root of the nerve. 



