774 



NERVOUS SYSTEM OF THE HORSE . 



minates in front by entering the base of the cerebellum, forming its posterior 

 peduncle. 



In the medulla the fiber tracts of the spinal cord either terminate in the nuclei of the gray 

 matter or undergo changes in their relative position, and new tracts appear. The gray matter 

 is highly modified and forms masses which have no homologues in the cord. The central canal 

 of the closed part of the medulla is surrounded by a thicker layer of gray matter than is the case 

 in the spinal cord. As the medulla opens out this gray matter is naturally spread in the floor of 

 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 the efferent or motor cranial nerves and the motor root-fibers 

 of the mixed nerves. Of the twelve pairs of cranial 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 dorsal to the pyramid on each side. On cross-sections it appears 



Ti'uiisverse fissure 

 - - — Ceriibellum 



Olfactory 

 bulb 



Olfactory 

 tract 



Optic nerve 



Infundi- 

 hulum 



Hypo- 

 physis 



Cerebral 

 peduncle 



Pons 



Medulla 

 oblongata 



Fig. 632. — Brain of Horse; Left ^'^EW. 

 I, Lateral fissure; S, suprasylvian fissure; 3, presylvian fissure; 4^ ectomarginal fissure; 5, 5', sulcus rhinalis an- 

 terior et posterior; L. p., piriform lobe; B, p., middle peduncle of cerebellum (brachium pontis) ; P. c, choroid plexus 

 of fourth ventricle. Roman numerals indicate roots of cranial nerves. 



as a wavy layer ot gray matter which is folded on itself so as to enclose 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 medial 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 the posterior 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 gracihs and nucleus cuneatus fibers arise which are traceable forward to the thalamus. 

 These are the internal arcuate fibers (Fibrae arcuata? intemae), which curve across the median 

 plane ventral to 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 continuation of the sensory conducting path from the dorsal roots of the spinal nerves. 

 The external arcuate fibers (Fibrae arcuatse extemae), some of which were seen on the lateral 

 aspect of the medulla, have a similar origin. Part of them (Fibrse dorsales) pass directly to the 

 dorsal aspect of the restiform body of the same side; others (Fibrse 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 addition to the arcuate fibers, the cerebello-olivary fasciculus before mentioned, 



