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 hes dorsal to the pyramid on each side. On cross-sections it appears 
Transverse fissure 
Cerebellum 
Olfactory 
bulb 
Olfactory 
tract 
Optic nerve / ~ 
Infundi- Hypo- Cerebral Pons Medulla 
bulum physis peduncle oblongata 
Fic. 632.—Brain or Horse; Lerr View. 
1, Lateral fissure; 2, 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 of 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 gracilis and nucleus cuneatus fibers arise which are traceable forward to the thalamus. 
These are the internal arcuate fibers (Fibre arcuatee interne), 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 (Fibre arcuate extern), 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 (Fibre: 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, 
