THE NERVOUS SYSTEM. 



105 



stimulus, the position at which it was applied as well as upon the condi- 

 tion of the nerve cells; the connection between the cells being so intimate 

 that a series of co-ordinated movements may result from a single stimula- 

 tion, first of all affecting one cell. Whether the cells possess as well the 

 power of originating impulses (automatism) is doubtful, but this is pos- 

 sible in the case of vaso-motor centres which are situated in the cord (p. 

 154, Vol. I.), and of sweating centres which must be closely related to 

 them, and possibly in the case of the centres for maintaining the tone of 

 muscles. 



THE MEDULLA OBLONGATA. 



The medulla oblongata (Figs. 321, 322), is a column of grey and white 

 nervous substance formed by the prolongation upward of the spinal cord 

 and connecting it with the brain. 



FIG. 321. 



FIG. 322. 



FIG. 321. Anterior surface of the pons Varolii, and medulla oblongata. a, a, anterior pyramids; 

 6, their decussation; c, c, olivary bodies; d, d, restiform bodies; e, arciform fibres; /, fibres described 

 by Solly as passing from the anterior column of the cord to the cerebellum; gr, anterior column of 

 the spinal cord; ft, lateral column; p, pons Varolii; i, its upper fibres; 5, 5, roots of the fifth pair of 

 nerves. 



FIG. 322. Posterior surface of the pons Varolii, corpora quadrigemina, and medulla oblongata. 

 The peduncles of the cerebellum are cut short at the side, a, a, the upper pair of corpora quadri 

 gemma; 6, ft, the lower; /,/, superior peduncles of the cerebellum; c, eminence connected with the 

 nucleus of the hypoglossal nerve ; e, that of the glosso-pharyngeal nerve ; t, that of the vagus nerve ; 

 d, d, restiform bodies; ;>, p, posterior pyramids; v, v, groove in the middle of the fourth ventricle, 

 ending below in the calamus scriptorius; 7, 7, roots of the auditory nerves. 



Structure. The grey substance which it contains is situated in the 

 interior, and variously divided into masses and laminae by the white or 

 fibrous substance which is arranged partly in external columns, and partly 

 in fasciculi traversing the central grey matter. The medulla oblongata 

 is larger than any part of the spinal cord. Its columns are pyriform, 

 enlarging as they proceed toward the brain, and are continuous with those 





