THE ISTHMVS. 685 



To this important system of white longitudinal fibres— & prolongation of 

 those of the spinal cord — is found annexed as complementary elements in the 

 organisation of the encephalic isthmus, several systems of transverse fibres — 

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

 exposition of the arrangement of these new elements. In proceeding from 

 behind to before, we notice, among the white transverse fihres : 



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

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

 corpus restif orme ; the inferior is buried in the intermediate fissure of the 

 pyramid and the lateral fasciculus. 



2. The proper fibres of the pons 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 

 medulla 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 

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

 of Vieussens. 



DIFFERENTIAL OHABAOTEES IN THE ISTHMUS OP OTHER THAN SOLffED ANIMALS. 



Apart from its volume, the isthmus does not present any sensible differences in 

 RumLanU and the Fig. In the Ox, it is remarked that : 1, The inferior Jyram^ds of the 

 medulla oblongata are more prominent, and the transverse cords parallel to the pons 

 Varolii more voluminous than in Solipeds ; 2, The crura cerebri are short ; o The optio 

 nerves are larger than in Solipeds; 4, There is a largely developed p«(wto-y 3 a,.d 

 excavated by a%vide cavity, and flattened from above to below; 5 Lastly the te.ies are 

 more conical and less distinct from the nates than m the animals already studied 



In the Carnimra, the fourth ventricle is very large and deep, and bordered by 

 salient and detached 'corpo/a restif ormia Its floor is marked ^r fXoMonllta ZX 

 striEB The vans Varoliiis large ; the cords (or columns) of the medulla oblongata paraiei 

 fs posterior border, are as diveloped as in the Horse, without taking into consideration 

 tL difference in size ^f the two species. The pyramids are voluminous, and the olivary 

 bodies well defined. The testes are larger than the nates. 



COMPARISON OF THE ISTHMUS OP MAN WITH THAT OP ANIMALS. 



In human anatomy, the medulla oblongata and encephalic isthmus are described 

 separately. ^^ .^^ ^^^^^ ^^^^ ^ well-marked groove, a continuation of that of the 



sninal medulla- it terminates anteriorly in a deep fossa named the /omraejj c^mm 0/ 

 Vi^ld-Zyr The pyramids are well marked. The olivary bodies are much more 



