68 



ESSENTIALS OF PHYSIOLOGY. 



and opens out into the fourth ventricle, and (2) by the appearance of 

 a lateral projection behind the pyramid, known as the olive, and con- 

 taining a folded sheet of grey substance internally, the olivary nucleus. 

 In the floor of the fourth ventricle are the nuclei of the hypoglossal 

 and of the vagus and glossopharyngeal nerves. Lateral to these nuclei 

 are the nucleus gracilis and nucleus cuneatus. Near the nucleus 

 cuneatus is the spinal (descending) root of the fifth nerve, and ventral 

 to the hypoglossal nucleus is the nucleus ambiguus. The interior of 



this part of the medulla ob- 

 longata is occupied chiefly 

 by the olivary nucleus and 

 the formatio reticularis. 

 The latter consists of nerve 

 fibres, some running trans- 

 versely and some, includ- 

 ing those of the fillet, run- 

 ning longitudinally. It also 

 contains some scattered 

 nerve cells. Ventral to the 

 formatio reticularis is the 

 pyramid, and lateral to the 

 nucleus cuneatus and the 



FIG. 18. Diagram of upper part of medulla spinal root of the fifth 

 oblongata. nerye ig a tract of nerye 



a, Pyramid; &. fibre from olivary nucleus; c, spino- n , .. 



thalamic fibres and Gowers' tract; d, restiform body; fibres, the restlform body 

 4, nucleus cuneatus;/, nucleus gracilis; g, nucleus of . . -i i f ,1 



vagus ; h, nucleus of hypoglossal nerve. or inferior peduncle of the 



cerebellum. The hypo- 

 glossal nerve crosses the formatio reticularis from its nucleus to emerge 

 in front of the olive, and the vagus and, at a higher level, the glosso- 

 pharyngeal nerve take a more lateral course through the formatio 

 reticularis to reach the surface behind the olive. Some of the internal 

 arcuate fibres, arising from nerve cells in the nucleus gracilis, nucleus 

 cuneatus, and olivary nucleus, cross the middle line to form part of the 

 restiform body. The external arcuate fibres are derived from the 

 gracile and cuneate nuclei and pass forward to the anterior median 

 fissure, where they sweep backward over the pyramid and olive of the 

 opposite side to join the restiform body (fig. 18). 



THE FUNCTIONS OF THE MEDULLA OBLONGATA. 



Like the spinal cord, the medulla oblongata acts as a reflex centre 

 or series of centres, and it also serves as a conducting path for impulses 

 passing between the brain and spinal cord. 





