THE NERVOUS SYSTEM • 513 



oblongata. Where the restiform bodies join the medulla, its wall projects 

 laterally to form the cerebellar flocculi. (Fig. 424) 



The cerebellar cortex consists of gray matter. Embedded in its 

 central core of white fibrous matter is a folded layer of gray matter, the 

 dentate nucleus. The Purkinje cells of the cerebellar cortex have synaptic 

 connexions with ascending sensory fiber tracts. From them impulses 

 pass to the cells of the dentate nucleus and thence by way of the brachia 

 conjunctiva into the tegmentum of the midbrain. 



A conspicuous feature of the ventral wall of the metencephalon of 

 mammals is the pons (Varoli), which has already been described as a 

 bridge of nerve fibers connecting the two hemispheres of the cerebellum. 

 Each lateral half of the pons becomes a middle cerebral peduncle or 

 brachium pontis. The fibers of the pons come largely from the cerebral 

 hemispheres, which decussate or cross to the opposite side as they pass 

 to the cerebellar hemispheres. The posterior boundary of the pons marks 

 the anterior limit of the myelencephalon or medulla oblongata. 



Medulla Oblongata. The non-nervous roof of the myelencephalon 

 or medulla oblongata of cyclostomes is modified as a chorioid plexus and 

 serves as a mechanism for nourishment of the brain. A similar plexus 

 appears in the medulla oblongata of all vertebrates. Cranial nerves V to 

 X have their roots and their motor centers in the lateral walls of the 

 medulla. A number of giant ganghon cells occur in the medulla of 

 cyclostomes, and, by their neurites or Mueller's fibers, connect with the 

 tail muscles. The main mass of the lateral and ventral wall of the medulla 

 consists of longitudinal fiber tracts of both ascending and descending 

 fibers, which connect brain and spinal cord. 



In Elasmobranchs, Ganoids, and Dipnoi the lateral walls of the 

 anterior part of the medulla form conspicuous "ears," the restiform 

 bodies, in which are equiUbratory centers, which correlate the sensory 

 nerves of the semicircular canals with trunk muscles. With the develop- 

 ment of lateral line organs, their nervous centers in the lateral walls 

 of the medulla form the conspicuous paired longitudinal vagus lobes. 

 But in amphibia and reptiles these vagus lobes disappear, in correlation 

 with the loss of the lateral line organs. 



The beginning of the medulla oblongata at the posterior boundary 

 of the pons is marked in the human brain by the origin of the abducens 

 nerve. Extending along the ventral surface of the medulla are the paired 

 P3n-amids, formed by bundles of fibers which connect the cerebral hemi- 

 spheres with the trunk. Lateral to the pyramids are two shorter swellings, 

 the oliva, which mark the position of the inferior olivary nucleus. 



Sections of the olivary nucleus show that, like the dentate nucleus 

 of the cerebellum, it is a horse-shoe shaped and crenated mass of gray 

 matter. Fiber tracts connect the olivary nucleus with the cerebellum. 



