TEE ISTHMUS. 767 



it increases in size from behind to before, and may be considered as having 

 ioxiT faces and ttvo extremities. 



The inferior face (Fig. 424), on which we can distinctly, and without any 

 preparation, perceive the natural limits of the isthmus, is crossed nearly in its 

 middle by a thick fasciculus of arciform fibres, which constitute the annular pro- 

 tuberance {protulerantia annularis), pons Varolii, or mesocephalon (or nodus 

 encephali). All the portion lying behind this fasciculus belongs to the medulla 

 oblongata. That in front forms the cerebral peduncles {crura cerebri). 



The superior face (Fig. 425), covered by the cerebellum and the posterior 

 extremity of the cerebral lobes, is more mammillated than the preceding. Passing 

 from behind to before — on the superior face of the medulla oblongata — there is 

 remarked the section of the peduncles of the ce^-ebellum, the vcdve of Vieussens, the 

 corpora quadrigemina, and the thalami optici. 



The lateral faces (Fig. 426), concealed in their anterior part by the hemi- 

 spheres of the brain, exhibit the profile of the medulla oblongata, pons Varolii, 

 peduncles of the cerebellum (crura cerebelli), cerebral peduncles (crura cerebri), 

 corpora quadrigemina, and thalami optici. 



The posterior extremity of the isthmus belongs to the spinal bulb, and 

 continues the spinal cord, from which it is only distinguished artificially. 



The anterior extremity is enveloped below, and on each side, by the oblique 

 fasciculi which form the two optic nerves, and beneath which are insinuated the 

 fibres of the isthmus before they pass into those parts of the cerebral hemispheres 

 which bear the name of corpora striata. 



After this enumeration of all the parts which constitute the isthmus of the 

 brain, we will examine them in detail, and in the following order : 1. Medulla 

 oblongata. 2. Po7is Varolii. 3. Crura cerebri. 4. Crura cerebelli. 5. Valve 

 of Vieussens. 6. Corpora quadrigemi?ia. 7. Thalami optici. After these we 

 will describe the pineal and pituitary glands — small appended lob^s placed, one on 

 the superior, the other on the inferior face of the isthmus.^ 



1. Medulla Oblongata (Figs. 424, 425, 426). 



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, flat- 

 tened above and below, and having four faces — an inferior, superior, and two lateral. 



Inferior face (Fig. 424). — 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 spinal cord. 



' There is far from being any agreement ms 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, nevertlieless, be found perfectly circumscribed if it be examined in the lower 

 animals, and particularly in the Horse. An antero-posterior section of the brain, made to one 

 side of the middle line, appears to us all that is needed to definitely settle the point. This 

 section, seen in Fig. 428, shows in the clearest 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 quadrifremina. and the thalami 

 optici. All these, then, belong to one and the same system— the medullary prolongation, which 

 serves as a bond of union between the three principal masses of the brain, and which we havu 

 designated the isthmus. It may be added, that this manner of considering the encephalic 

 isthmus perfectly agrees with the teachings of physiology. 



