676 



THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



glands : small appended lobes placed one on the superior, the other on the 

 inferior face of the isthmus. 1 



Fig. 322. 



Medulla OUongata. (Figs. 323, 

 324, 329.) 



The medulla oblongata constitutes 

 the posterior portion of the encephalic 

 isthmus ; it succeeds the spinal cord, 

 and extends forward as far as the pons 

 Varolii. It is a thick peduncle of a 

 white colour, wider before than behind, 

 flattened above and below, and having 

 four faces an inferior, superior, and 

 two lateral. 



Inferior face (Fig. 322). This 

 face rests in the channel of the basilar 

 process. Convex from side to side, 

 and limited anteriorly by a transverse 

 fissure which separates it from the 

 pons Varolii, posteriorly it does not 

 offer anything to distinguish it from 

 the medullary axis. 



On the middle line there is a well- 

 marked fissure, a continuation of the 

 inferior fissure of the cord, which lies 

 between two very elongated promi- 

 nences that are sometimes but little 

 apparent, and from their form are 

 named the pyramids of the bulb (corpora 

 pyramidalia) (Figs. 322, 19 ; 338, 6). 

 The base of these pyramids touches 

 the pons Varolii, and their apex is 

 insensibly lost, posteriorly, on reach- 

 ing the spinal cord. 



Outwardly is an almost plane sur- 

 face, bordered anteriorly by a trans- 

 verse band which lies immediately 

 behind the pons Varolii ; sometimes 

 it is covered for the greater part of its 

 extent by a very thin expansion of 

 arciform fibres, between the anterior 

 border of which and the transverse 



GENERAL VIEW OF THE BRAIN; LOWER 

 SURFACE. 



1, Olfactory lobe ; 2, Cavity of the olfactory 

 lobe ; 3, External root of olfactory lobe ; 

 4, 5, Cerebral hemispheres ; 6, Cerebellum ; 

 7, Optic chiasma, or commissure ; 8, Pitui- 

 tary gland; 9, Optic nerves; 10, Tuber 

 cinereum; 11, Crus cerebri; 12, Third 

 cranial nerve ; 13, Fourth nerve ; 14, Pons 

 Varolii ; 15, Fifth nea-ve ; 16, Sixth nerve ; 

 17, Seventh and eighth nerves; 18, Me- 

 dulla oblongata, the number being placed 

 on the olivary body ; 19, Anterior pyramid ; 

 20, Roots of ninth, tenth, and eleventh 

 nerves ; 21, Twelfth nerve. 



1 There is far from being any agreement as to the number of parts which ought to 

 compose the encephalic isthmus, some authorities making more, some less. The limits 

 of this small apparatus^will, nevertheless, be found perfectly circumscribed if it be ex- 

 amined in the lower animals, and particularly in the Horse. An antero-posterior section 

 of the encephalon made to one side of the median plane appears to us all that is needed 

 to definitely settle the point. This section, seen in figure 329. shows in the plainest 

 manner that the encephalic prolongation of the spinal axis extends to the corpora 

 striata, and that it comprises the medulla oblongata, pons Varolii, cerebral and cerebellar 

 peduncles (or crura), the corpora quadrigemina, and the thalami optici. All these, then, 

 belong to one and the same system the medullary peduncle, which serves as a bond 

 of union between the three principal masses of the encephalon, and which we have 

 designated the isthmus. It may be add( d that this manner of considering the encephalic 

 isthmus perfectly agrees with the teachings of physiology. 



