NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPHALON.) 



679 



aUongee, bulbe rachidlen. Germ, das Verlan- 

 gerte Mark. Ital. midollo allungato.)We 

 begin with the description of this segment be- 

 cause of its immediate connection with the 

 spinal cord, for it is plain, since this is the 

 connecting link between that centre and the 

 intra-cranial mass, that whatever influence the 

 latter may exercise upon the former, must be 

 conveyed or propagated by the medulla ob- 

 longata. 



It is proper to notice that the term medulla 

 oblongata has not been employed in a uniform 

 sense by all anatomists. Willis and Vieussens 

 comprehended under this title all the parts 

 from the corpora striata and optic thalami 

 (both included) down to the commencement 

 of the spinal cord.* The same signification 

 was adopted by the writers who immediately 

 followed these great anatomists. Winslow con- 

 siders the medulla oblongata as " one middle 

 medullary basis common to both cerebrum and 

 cerebellum, by the reciprocal continuity of 

 their medullary substances." f The crura, or 

 pedtmculi cerebri, constitute its anterior part : 

 these seem to be lost in the corpora striata, as 

 Winslow states, and therefore they are looked 

 upon as the peduncles of the cerebrum. Its 

 posterior portion is called the extremity or 

 cauda of the medulla oblongata (queue de la 

 moelle allongee). It is to this latter portion that 

 Haller restricted the term medulla oblongata, 

 and most modern anatomists follow his example. 

 Itolando, however, still applies the term in its 

 more extended sense. 



In the present article, we adopt the phrase- 

 ology of Haller as far as regards the term me- 

 dulla oblongata. It seems to form an upper 

 enlarged portion of the medulla spinalis, to 

 which it stands in somewhat the same relation as 

 the capital to the shaft of a column. Its superior 

 limit is indicated by the posterior edge of the 

 ponsVarolii; its inferior is denoted by a horizon- 

 tal plane extended between the occipital foramen 

 and the first vertebra. A more natural line of 

 demarcation, however, between this part and 

 the medulla spinalis may be found in certain 

 decussating fibres which are seen crossing the 

 anterior median fissure of the former at its infe- 

 rior extremity. No such limit as this, however, 

 is found on the posterior surface (fig. 383). 



The medulla oblongata has somewhat of a 

 conical shape, its base being situate above at 

 the posterior margin of the pons. It is slightly 

 flattened on both anterior and posterior sur- 

 faces, more so on the latter than on the former. 

 The medulla oblongata admits of the same pri- 

 mary subdivision as the medulla spinal is, namely, 

 into two equal and symmetrical portions sepa- 

 rated from each other by an anterior and a poste- 

 rior median fissure. The former is wide but not 

 of great depth. It is occupied by a fold of pia 

 mater. Its floor is formed by a layer of fibrous 

 matter which has the same cribriform appear- 

 ance as that of the anterior spinal fissure. 

 These fibres are commissural, connecting the 



* See the quotation from the English edition of 

 Wil is, at p. 669. 



t Winslow's Anatomy, translated by Douglas, 

 vol. ii. p. 316. Edin. 1763. 



Fig. 383. 



pair 



Anterior view of the medulla oblonyata and pons 

 Varolii. (After Arnold.) 



a, anterior extremity of the pons. 



p, anterior pyramids. 



d, decussating fibres of anterior pyramids. 



o r olivary bodies. 



A, arciform fibres. 



D, portio dura 1 Seventh pair 



I, portio intermedia of Wrisbcrg f o{ nerv g s< 



M, portio mollis 



G, glosso-pharyngeal nerve \ Eighth 



V, par vagum * 



S, spinal accessory 



two portions of the medulla oblongata. The 

 posterior fissure is very deep and narrow. It is 

 not limited in front by a grey commissure as the 

 posterior spinal fissure is, but by the posterior 

 surface of the white commissure just described. 

 A single layer of the pia mater passes into it. 

 The continuity of the anterior fissure of the me- 

 dulla oblongata and of that of the spinal cord is 

 interrupted by the decussating fibres of the py- 

 ramids, (fig. 383, d,) but the posterior fissures 

 are distinctly continuous with each other. 



On either side of the median plane there are 

 indications on the surface of the medulla ob- 

 longata, which suggest a subdivision of each 

 half of the organ into four columns of nervous 

 matter, through the medium of which it forms 

 its connection with certain parts of the cere- 

 brum and cerebellum on the one hand, and of 

 the spinal cord on the other. These columns 

 are the anterior pyramidal, the olivary, the 

 restiform, and the posterior pyramidal. 



The anterior pyramidal columns, or anterior 

 pyramids, (fins. 383, 384, 385, p,) are two 

 prismatic bundles of fibrous matter which 

 extend between the antero-lateral columns of 

 the spinal cord and the lateral hemispheres 

 of the brain. In the medulla oblongata 

 each of these columns forms a compact body, 

 winch, when cut transversely, exhibits a tri- 

 angular outline in its central portion, but that 

 of a cylinder at either extremity. Each pyramid 

 is limited on the outside by a superficial groove, 

 which separates it from the olivary column, 

 and on the inside by the anterior median fis- 



