Medulla Oblongata 55 



Though the pons consists chiefly of white fibres, it contains also 

 grey matter, and helps in co-ordinating muscular movements. There 

 also decussation of the fifth and of the seventh nerves takes place. If a 

 lesion, a small haemorrhage, for instance, occur above the crossing of 

 the facial fibres, there is paralysis of the opposite sides of face and of 

 the body, whereas if it be below the crossing the facial paralysis is 

 upon the same side as the lesion, whilst the hemiplegia is upon the 

 opposite side, for the motor fibres of the cord cross in the medulla. 



Haemorrhage into the pons is usually accompanied by contraction 

 of the pupils. 



The medulla oblong ata extends from the lower border of the pons 

 to the ring of the atlas. Its anterior surface lies upon the basilar 

 process of the occipital bone, and its posterior surface rests between 

 the hemispheres of the cerebellum. Being an enlargement of the 

 spinal cord, it consists of symmetrical lateral halves in which the 

 various columns of the cord can be traced. In front is the median 

 groove, which ends at the pons as the foramen caecum, and the posterior 

 median groove becomes widened out into the fourth ventricle, the 

 grey commissure of the cord being exposed in its floor. The anterior 

 columns of the cord are represented by the anterior pyramids ; the 

 bundles of fibres which decussate at the lower part of the groove 

 between the pyramids actually belong to the lateral columns they 

 are called the crossed pyramidal tracts. The outermost fibres of the 

 pyramids do not cross they constitute the direct pyramidal tracts. 

 (See SPINAL CORD, p. 213.) 



The olive belongs to the lateral column. The restiform bodies 

 (restis, rope) are cord-like bundles of fibres continuing the posterior 

 column into the cerebellum ; they bound the fourth ventricle. The 

 posterior pyramid continues Goll's column along the median border 

 of the restiform body, and its fibres gradually pass into the restiform 

 body. In the exposed grey matter of the fourth ventricle descends a 

 narrow median groove, which at the apex of the ventricle was origi- 

 nally continued into the central canal of the cord. 



The medulla oblongata is described by Ranney as ' the true nerve- 

 centre of animal life ' ; all the cranial nerves from the seventh to the 

 twelfth arise directly from it, whilst others can also be traced thither, 

 that is, to the floor of the fourth ventricle. It contains much grey 

 matter : its action, therefore, is ' largely reflex. 3 Taking the nerves 

 arising from it in numerical order, we find the seventh controlling the 

 reflex movements of the facial muscles ; the ninth (glosso-pharyngeal), 

 of deglutition ; the tenth (pneumogastric), of respiration ; the eleventh 

 (spinal accessory), of the larynx and heart ; and the twelfth (hypo- 

 glossal), of the tongue. 



The special centres of the medulla are respiratory, under the 

 influence of the pneumogastric, trifacial, and other afferent fibres. 

 * This centre also presides over the acts of laughing, sighing, sobbing, 



