TEE ISTHMUS. 675 



raises the pituitary gland, the cerebral lobes, and the cerebellum. Then we pass to the 

 corpus callosum, which is exposed, as in figure 330, by a horizontal section of the hemi- 

 spheres across the centrum ovale. The corpus callosum of each side is afterwards 

 excised on the median line to reach the interior of the lateral ventricles, and this great 

 commissure of the brain ought, after studying the septum lucidum, to be cut across in the 

 middle and turned over, as in figure 331, so as to show the cerebral trigonal (fornix). The 

 foramen of Monro is next examined, then the corpus striatum, hippocampi, tsenia semi-' 

 circularis, choroid plexus, and velum interpositum, which are expused by the ablation of 

 the hippocampi and trigonal. Lastly, we return to the foramen of Monro to study its 

 communication with the ventricle of the optic layers ; it will be well, also, to again ex- 

 amine the latter, as well as the aqueduct of Sylvius and the ventricle of the cerebellum, 

 which we arrive at in dividing the organ through the middle and separating the halves. 



Two longitudinal and vertical sections, one median (Fig. 327), the other at the side 

 (Fig. 329), will not be without utility in the study of these particulars. They may be 

 made by means of a saw, the brain remaining inclosed in the cranial cavity. 



(A long-useful implement for removing the bony casing of the brain without risk of 

 injuring it, is a chisel whose thin cutting edge is slightly concave, the corners being 

 smooth and rounded, and projecting beyond the cutting edge.) 



ARTICLE II. THE ISTHMUS. 



We will study in succession the external and internal conformation of 

 this organ, and its structure. 



EXTERNAL CONFORMATION OF THE ISTHMUS. 



The isthmus is a prismatic prolongation of the spinal cord supporting the 

 cerebellum, and terminating in the cerebral hemispheres ; it increases in 

 size from behind to before, and may be considered as having four faces and 

 two extremities. 



The inferior face (Fig. 322), 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 protuberance (protuberantia anmdaris), pons Varolii, or mesocephalon 

 (or nodus encephali). All the portion lying behind this fasciculus belongs 

 to the rachidian bulb (bulbus rachidicus or medulla oblongata). That in front 

 forms the cerebral peduncles (crura cerebri). 



The superior face (Fig. 323), covered by the cerebellum and the 

 posterior extremity of the cerebral lobes, is more mammillated than the pre- 

 ceding. Passing from behind to before, on the superior face of the medulla 

 oblongata, there is remarked the section of the peduncles of the cerebellum, the 

 valve of Vieussens, the corpora quadrigemina, and the optic layers (thalami optici). 



The lateral, faces (Fig. 324), concealed in their anterior part by the 

 hemispheres 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 medulla oblongata, 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 that part of the 

 cerebral hemispheres which bears the name of corpora striata. 



After this enumeration of all the organs whose aggregation constitutes 

 the isthmus of the encephalon, we will examine them in detail, and in the 

 following order : 1, Medulla oblongata ; 2, Pons Varolii ; 3, Crura cerebri ; 

 4, Crura cerebelli ; 5, Valve of Vieussens ; 6, Corpora quadrigemina ; 

 7, Thalami optici. After these, we will describe the pineal and pituitary 



