776 NERVOUS SYSTEM OF THE HORSE 
posterior grooves. Viewed ventrally, it is elongated transversely, convex in 
both directions, and presents a wide shallow median groove (Sulcus basilaris), 
which lodges the basilar artery. Laterally a large part of its mass curves dorsally 
and backward into the base of the cerebellum, forming the middle cerebellar 
peduncle (Brachium pontis). The superficial origin of the trigeminal (fifth) nerve 
is at the lateral limit of the ventral surface. Transverse striations indicate the 
course of superficial (ventral) fibers which connect the two sides 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 raphé of the medulla is continued into 
the pons, dividing it into similar halves. In the anterior part of the pons the fillet divides into 
a medial and a lateral 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 hes close to the raphé under the 
gray matter of the floor of the fourth ventricle. In cross-section the formatio reticularis forms 
a large area below the superficial gray matter and the longitudinal bundles. Dorso-laterally is 
the large rounded section of the anterior cerebellar peduncle. Lower down is a large bundle, the 
sensory root of the fifth nerve. In front of this is the motor nucleus of the same nerve, lateral 
to which is its motor root. The basilar or ventral part of the pons (Pars basilaris pontis) is com- 
posed 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 dorsally 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 intercross. 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 raphé and run forward and backward in the brain stem. Others are axones 
of cells of the nuclei pontis, and pass to the hemispheres of the cerebellum. The corpus trape- 
zoideum is mainly the central continuation of the cochlear division of the acoustic nerve. Above 
it is the small anterior olivary nucleus (Nucleus olivaris 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 (basis) of the cerebral peduncles and are situated laterally, interspersed 
among the deep transverse fibers in the anterior part of the pons. Toward the posterior part the 
bundles 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 corticopontile fibers. 
THE CEREBELLUM 
The cerebellum is situated in the posterior fossa of the cranium, and is sep- 
arated from the cerebral hemispheres by 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 dorsally 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 (Velum medullare orale), a thin lamina which forms the anterior part of the 
roof of the fourth ventricle. The posterior extremity, the nodulus, gives attach- 
ment to the posterior medullary velum (Velum medullare aborale), which covers the 
posterior recess of the fourth ventricle. The hemispheres (Hemispheria cerebelli) 
