Till-: I+THMU8. C85 



TII tin's important system of while lomjifnilinal fibreg a prolongation of 



th. M of tin spinal cord is found annoxod as complementary elements in the 



:iis.ition of the encephalic isthmus, several systems of transverse fibres 



also white and masses of grey substance. The following is a summary 



-it ion of the arrangement of these new elements. In proceeding from 



bi-liind to before, we notice, among the while transverse fibres : 



1. The expansion of arciform fibres which sometimes covers the inferior 

 of the bulb (Fig. 338, /): their superior extremity is lost in the 



corpus restiforme; the inferior is buried in the intermediate fissure of the 

 pyramid and the lateral fasciculus. 



2. The proper fibres of the pous Varolii : they constitute a very thick 

 semicircular fasciculus whose extremities form the middle cerebellar 

 peduncles and enter the cerebellum ; this fasciculus envelops, inferiorly 

 and laterally, the longitudinal fibres of the isthmus ; it is crossed by several 

 superposed planes of transverse fibres. 



3. The transverse fibres of the valve of Vieussens and those of the white 

 commissures, which have been already noticed. 



The grey substance of the isthmus, which now remains to be mentioned, is 

 far from being so abundant as the white substance, and, as in the spinal 

 cord, it is principally deeply buried in the texture of the organ. In the 

 inrtlullsi oblongata none is found on the track of the superior and inferior 

 fasciculi of fibres ; but the lateral fasciculi are intermingled with it, 

 and there is a layer on the floor of the fourth ventricle. It is also found 

 in the cerebral peduncles, and particularly in the prolongation of the lateral 

 fasciculi of the bulb. 



Each of the corpora quadrigemina is composed of a small mass of this 

 grey substance, and is covered by a thin pellicle of white matter which is 

 scarcely perceptible in the anterior eminences. The optic thalamus is a 

 similar mass, though more voluminous, darker coloured, and without a layer 

 of white substance on its superficial face. 



Lastly, nerve cells also exist between the various layers of transverse 

 tilnvs of the pons Varolii, and between the tubes which constitute the valve 

 of Yi.-iis- 



I'UKKKKVI IAI. i HAKAi.TKKS IS llti: ISTHMUS OF UTHEU THAN SOUI'ED AXIM \ 



irt frtn its volume, tin- i>thnm.s docs nut present tiny sensible difiereiiee.- in 

 Ilmitiiitint" and the I'iij. In the <>je,\i i.- remarked that: 1, The iujtrivr i>yruinhl* <>f tl 



" i n!il.,n'j itn nro more prominent, and the transverse cords paralli 1 l th- | * 



Varolii more voluminous than in Sli|n"ls; "2, The n-nrti certbri are short; "t, The (>/>tii- 

 nerve* are lnr_rer than in Solids: I. Th- ie is a largely dt-velujx-d pituitary <jl<ni<l, 

 it-d hy a wide cavity, and llattened from uU.ve to In-low; ."i, I astly. the testes are 

 onieal and l -.- di.-tinet from tin- nutm than in the animal* already Mudinl. 

 In the Curiiirnr-i, th>- fourth r- n' i : lar^'e and deep, and l>rdered l>y 



>alii ut an: ^rjxtra rettiformin. Its tloor is marked l>y .- m<- \N hite Iran 



Thf inf I './<./// i.- lar^e ; the ,t>r<lf or rliiiitiii>) uf the iimlnlln ohloii'jula, parallel 

 :r,r liord-T, !.'jH-l an in the Horse, \vithout taking into eon.-ideration 



the dill'.-rein-i- in .-i/( of the two *|x ei. s. The iujrniniil* ar<- VulumiiuuiM, and the ulirary 

 .- well delined. The /->,' x are lrj,'er than tlie H<I/X. 



COMI-AIil-oN OK IMF. I8THMIS OK MAS WITH THAT Of ANIMALS. 



Ill huiua-i anatomy, the i,ilnl!n o'///;/<i/n and . lhiuu$ arc dttjor 



eparud ly. 



i it-, I,,\MT face a well marked L" inuation of that of tho 



>-jiinal iiniliilla: it terminates anteriorly in a deep !'".-> i mum d the i<>r<tun n fi >. 

 \i'-'/-<l Azi/r. The jii/riimiiln are well n.nrked. The i-l.rnnj I- 



