698 THE CENTRAL AXIS OF TEE NERVOUS SYSTEM. 



peripheral extremity, on the inner face of the grey covering of the con- 

 volutions. 



The fibres of the white substance of the hemispheres are connected with 

 those of the encephalic isthmus, through the latter being prolonged into the 

 texture of the corpus striatum, where they appear either in the form of striae, 

 or as an elongated nucleus, known as the double semicircular centre of Vieus- 

 sens ; they then pass to the outside of the ventricular cavity, and plunge into 

 the centrum ovale of the hemisphere, where they are manifestly continued by a 

 portion of the fibres constituting it. It has been said that these fibres, instead 

 of thus disappearing in the hemisphere, ascend at first to the right and left 

 on the external side of the lateral ventricle, and are afterwards inflected in- 

 wards above that cavity, to join on the median line ; and in this way form the 

 corpus callosum. I have searched for this arrangement in our domesticated 

 animals, and particularly in the Dog, whose brain is well adapted for study- 

 ing the corpus callosum, but without success. It has always appeared to 

 me that the extremities of the transverse fibres which form this great com- 

 missure are lost in the white substance of the hemispheres, some passing 

 above, the others below ; and I believe I have also seen some of the pedun- 

 cular fibres radiating in the centrum ovale becoming insinuated between the 

 extremities of the fasciculi of the corpus callosum, to gain the superior part 

 of the hemisphere, without being continuous in any way with these fasciculi. 



Some of the nerve-tubes certainly terminate in the corpus callosum; 

 but in this there is nothing extraordinary, as in that layer there are nuclei 

 and some nerve-cells. 



DIFFEEENTIAL CHAEACTERS OF THE CEREBRUM IN OTHER THAN SOLIPED ANIMALS. 



The brain, in the animals now referred to, offers some differences in volume, as might 

 be inferred from what has been said regarding its relations with the cerebellum. In all, 

 its development posteriorly is not so considerable that it covers the latter, which always 

 remains exposed. 



Its general form varies a little. In the Ox, the hemispheres are proportionately 

 larger posteriorly than in the Horse, but contract suddenly at the fissure of Sylvius, 

 preserving their reduced dimensions in the anterior lobes ; the latter are therefore more 

 conical than in Solipeds. The cerebrum of the Dog is regularly ovoid, except at ihe 

 extremity of the anterior lobes ; there the hemispheres become much flattened from one 

 side to the other, and form a kind of spur that enters the ethmoidal fossae. 



The cerebral convolutions are a little larger in the Ox than in the Horse, but they are 

 also less numerous ; they are still fewer in the Pig, and yet less in the Carnivora ; we 

 will see hereafter that they are absent in Birds. The particular features of the lower 

 face of the hemispheres are the same in all the species; except that the olfactory lobes 

 are more detached than in Solipeds ; they are remarkably developed in the Dog. 



The ventricles are the same in all ; the floor is always formed by the corpus striatum, 

 hippocampus, and thalami optici, the roof by the corpus callosum. In the Ox, the band 

 of the hippocampus is remarkable for its width ; in the Dog, the corpus striatum, pro- 

 portionately voluminous, is of a deep grey colour on its surface. 



COMPAEISON OF THE CEREBRUM OF MAN WITH THAT OF ANIMALS. 



The cerebrum of Man (Fig. 333) is distinguished by its regularly ovoid shape, and 

 its great development, particulary behind, where it covers the cerebellum a feature 

 never observed in animals. 



Viewed superiorly, a cerebral hemisphere is clearly divided into three lobes: an 

 anterior or frontal; a middle or spnenoidal, corresponding to the mastoid lobule of the 

 Horse; and a posterior or occipital, covering the cerebellum. The two first are separated 

 by a narrow, deep, and sinuous fissure cf Sylvius. 



The convolutions are larger, and separated by deeper fin-rows, than those of the 

 Horse, but they are not more numerous. The olfactory lobes arise, as in animals, from 

 two orders of roots, but they are small and entirely hidden beneath the inferior face of 

 the frontal lobes. 



