ANATOMY AND PHYSIOLOGY OF THE BRAIN 357 



cord (motor or volitional), and others give off axones 

 which form portions of the cranial nerves. 



The white substance is composed of bundles of nerve 

 fibers supported by a frame-work of neuroglia and 

 connective tissue. These columns formed of bundles 

 of nerve fibers are the connecting or conducting 

 pathways coursing from the brain to the spinal cord, 

 for the transmission of nerve impulses between the 

 brain and the periphery. 



The Pons Varolii. The pons is a white mass on the 

 anterior aspect of the brain stem placed between the 

 medulla oblongata and the crura cerebri. It is convex 

 from side to side, containing mostly transverse and 

 longitudinal fibers. The transverse fibers are collected 

 into rounded bundles, to continue as the middle 

 peduncles into the white substance of the correspond- 

 ing cerebellar hemispheres. The middle peduncles are 

 commissural paths consisting of axones coursing in 

 opposite directions connecting the nuclei with the 

 cerebellum; then some axones pass into the opposite 

 middle peduncle, forming uninterrupted commissural 

 systems; again, a few fibers communicate with nuclei 

 in the brain stem, notably the oculomotor, trochlear, 

 and abducent cranial nerves. 



The Structure of the Pons Varolii. It consists of a 

 central gray and white mass of nerve tissue, supported 

 by neuroglia and connective tissue. Microscopically, 

 bundles of nerve fibers and nerve cells can be seen 

 in groups, the former coursing in a longitudinal and 

 transverse direction, as continuations of the pathways 

 of nerve fibers from the cord and medulla below, 

 and the cerebrum and cerebellum above. The trans- 

 verse bundles of nerve fibers in the pons convey 

 impulses from the corresponding and opposite hemi- 

 spheres of the brain. The nerve cells in the pons give 

 off axones which form some of the cranial nerves. 



Functions of the Medulla Oblongata and Pons 

 Varolii. The medulla and pons contain tracts of 



