Till: l^THMVS. 675 



tin- pituitary ir'and. the eerehral lolies. u iul tl.i- << n l-ellnm. Then we paea to the 

 corpus callo>um. \\hieh N eX|>o.ed. ;is in tigim- '.''.',(), by a lim i.;it.il -ert.i.n of tin- hemi- 

 spheres across the centrum ovule. Tin- corpus rational of each .-ide is afterward* 

 .1 on tiie iiinl.iiM line to reach tin- interior of tin- laleiul v. utricles, and this great 

 commissure of the brain ought, alter studying the septum liu'iiluiu. to be cut across in the 

 middle anil tunic 1 o\< r. as in figure ',\'M, so as to show the cerebral trigi-nal fornix). The 

 foramen of Monro is n \t c.xamintd, th<'ii the corpus striatuin, hip|>ocniii[>i, t:eni;i semi- 

 circul .n is, clmroid pVxus, and velum interpositnm, whit li are exposed by the alilatioii of 

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

 coiiiiiitinii'iition with the ventricle of the optic livers; it will be well, also, to again ex- 

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

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



T\\" longitudinal and veitical .-e-'tious, one median (Fig. >-~), the otht r at the side 

 (Fig. '.'<'2-> . will not lie without utility in the study of the>e particular.-. Tin y may be 

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



(A loni;-u-ei'ul implement for removing the lioiiy casing of the lir.dn without risk of 

 injuring it, is a chi.-el whose thin cutting edge ia s-ligl.tly concave, the corners being 

 tli 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 

 iiniinlni- protuberance (protuberantia annularis), pons Varolii, or mesoceplialan 

 (or nidus 'HffjiJtiili i. All the portion lying behind this fasciculus belongs 

 t'> tin- nn-hitlinn lnll> (bnlbus racliidicw or medulla oblongata). That in front 

 leg (crnra cerebri). 



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



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



rt -ling. Passing from behind to before, on the superior face of the medulla 



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



"/ Vieussens, the corpora quadrigemina, and the optic layers (thalami opt if i). 



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

 hemispheres of tliu brain, exhibit the profile of the medulla oblongata, pons 

 Van ilii, peduncles of tho cerebellum (crwro cenltUi \, cerebral peduncles 

 i cerebri), corpora qoadrigemioa, and thulium optici. 



The_p*/<v/.' ;'.///' ///////of tin- isthmus belongs to tin- iiu-<lull:i oblongata, and 

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



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

 olilifjiu' fasciculi which form the two optic nerves, and beneath which are 

 insinuated the fibres of the isthmus before they pass into that part of tho 

 .il ln'iui-plicrcs which bears the name of corpora *//////<;. 



After tin's enumeration of all the organs whoso aggregation i-'iistitui. s 



tlie isthmus of the cnccphalon, \vi; will examine them in d< tail, ami in tho 



following order: 1, Medulla i>l>li,n>i<itu ; 2, P<m$ Varolii; 3, Crurti <->n!>i-i; 



'i-ura cerebelli ; 5, Valve of !"/ ' /" gWMfcljpmNM j 



7. Tlinlami optici. After these, we will describe the y//c ! and ]>ituitnr</ 



. J 



