660 NERVOUS SYSTEM OF THE HORSE 



lateral limit of the ventral surface. Transverse striations indicate the course of 

 its superficial (ventral) fibers which connect the two sitles of the cerebellum. The 

 dorsal surface is blended on either side with the overlying anterior peduncles of the 

 cerebellum ; the central free portion forms the anterior part of the floor of the fourth 

 ventricle, and will be considered in the account of that cavity. 



On cross-section the pons is seen to be composed of dorsal and ventral parts. The dorsal 

 part (Pars dorsalis pontis) consists superficially of a layer of gray matter covered by the epen- 

 dyma of the fourth ventricle. Beneath this the median raphe of the medulla is continued into 

 the pons, dividing it into similar halves. In the anterior part of the pons the fillet divides into 

 an inner and outer part, the medial and lateral fillets (Lemniscus medialis, lateralis); the latter 

 arches outward to reach the outer side of the anterior cerebellar peduncle. The dorsal longitudinal 

 fasciculus becomes sharply defined into a round bundle which lies close to the raplie imder the 

 gray matter of the floor of the fourth ventricle. In cross-section the formatio reticularis forms 

 a large area l)e!ow the superficial gray matter and the longituflinal l)undl(>s. Dorso-laterally is 

 the large rounded section of the anterior cerebellar peduncle. Lower down is a large Inmdle, the 

 sensory root of the fifth nerve. In front of tliis is the motor nucleus of the same nerve, exter- 

 nal to whicli is its motor root. The ventral part of the pons (Pars Imsilaris pontis) is composed 

 of transverse and longitudinal fibers, and a large amount of gray matter which is broken up into 

 small masses (Nuclei pontis) by the intersection of the fibers. The transverse fibers are gath- 

 ered laterally into a compact mass which turns upward and backward and enters the central white 

 matter of the cerebellum, forming the middle cerebellar peduncle. Centrally the fibers are ar- 

 ranged in bundles which intercrcss. The transverse fibers are chiefly of two kinds. Some arise 

 from the Purkinje cells of the cerebellar cortex and pass either to the opposite side of the cere- 

 bellum or turn at the raphe and run forward and Ijackward in the brain-stem. Others are ax- 

 ones of cells of the nuchn pontis, and ]iass to the hemispheres of the cerebellum. The corpus 

 trapezoideum is mainly the central continuation of the cochlear division of the auditory nerve. 

 Above it is the small anterior olivary nucleus (Nucleus oli^•aris nasalis). The longitudinal 

 fibers of the ventral part of the pons consist chiefly of the cerebrospinal or pyramidal fasciculi. 

 These come from the ventral part (pes) of the cerebral peduncles and are situated laterally, 

 interspersed among tlie deep transverse fibers in the anterior part of the pons. Toward the poste- 

 rior part the l^undles incline toward the median plane and become collected into a compact mass 

 which appears superficially at the posterior border. Many fibers come from the cerebral cortex 

 and terminate in the nuclei of the gray matter of the pons; they may be designated cortico- 

 pontile fibers. 



THE CEREBELLUM 



The cerebellum is situated in the posterior fossa of the cranium, and is sep- 

 arated from the cerel)ral hemispheres ])y the transverse fissure and the tentorium 

 cerebelli which occupies it. It overlies the pons and the greater part of the medulla, 

 from which it is separated by the fourth ventricle. Its average weight is about two 

 ounces (ca. 60 gm.) or about 9 per cent, of the weight of the entire brain. Its 

 shape is approximately globular but very irregular. It is somewhat compressed 

 dorso-ventrally and its transverse diameter is the greatest. 



The anterior surface faces upward and forward and is covered partially by 

 the tentorium cerebelli. The posterior surface is almost vertical. The ventral 

 surface or base lies over the fourth ventricle, and is connected by three pairs of 

 peduncles with the medulla, pons, and mid-brain. 



It is customary to recognize three gross divisions of the cerebellum, viz., the 

 median vermis and two lateral hemispheres. The vermis (cerebelli) is curved in a 

 circular manner so that its two extremities are close together or even in contact 

 on the ventral surface. The anterior extremity is termed the lingula; it lies 

 between the cerebellar peduncles and gives attachment to the anterior medullary 

 velum, a thin lamina which forms the anterior part of the roof of the fourth ven- 

 tricle. The posterior extremity, the nodulus, gives attachment to the posterior 

 medullary velum whicli covers the posterior recess of the fourth ventricle. The 

 hemispheres (llcmisplueria cerebelli) are clearly separated from the vermis bj^ 

 two deep paranunlian fissures. They lie in the lateral depressions of the cerebellar 

 compartment of the cranium. 



In tracing the fissures from beliind forward it will be noticed that they are nearly sagittal 

 as far as the anterior surface, where they diverge widely, so that the vermis forms all of the fore 

 part of the cerebellum. 



