648 THE NEKVOUS SYSTEM. 



fcion of the corpus callosum). It thus comes about that every part of the cerebral 

 cortex, with the exception of the bulbi olfactorii, the olfactory parts of the hemi- 

 sphere, and the inferior and anterior part of the temporal lobe, is reached by the 

 callosal fibres. But it should be clearly understood that all the regions of the cortex 

 do not receive an equal proportion of fibres ; in other words, some cortical areas 

 would appear to be more plentifully supplied than others. Another point of some 

 importance consists in the fact that the callosal fibres do not, as a rule, connect 

 together symmetrical portions of the gray cortex. As the fibres cross the median 

 plane they become greatly scattered, so that dissimilar parts of the cortex of 

 opposite hemispheres come to be associated with each other. 



The commissura anterior is a structure supplemental to the corpus callosum, 

 although originally it was the principal cerebral commissure long before the corpus 

 callosum was evolved. It connects together the two olfactory bulbs, and also 

 portions of the opposite temporal lobes. It presents a cord-like appearance and 

 in median section appears as a small oval bundle in the lamina terminalis 

 (Fig. 544, p. 617). The middle free portion is placed immediately in front of the 

 columns of the fornix as they curve downwards, and also in intimate relation to the 

 anterior end of the third ventricle. Posteriorly, the small portion of the anterior 

 commissure which appears in the ventricle between the two columns of the fornix is 

 clothed with the ventricular ependyma ; anteriorly, the commissure is connected 

 with the lamina terminalis as it stretches from the optic chiasma upwards towards 

 the inferior (anterior) end of the hippocampal commissure. 



The lateral part of the anterior commissure penetrates the cerebral hemisphere, 

 and, gaining the inferior part of the anterior end of the internal capsule, divides into 

 two portions, viz., a small inferior olfactory part and a much larger temporal part. 



The olfactory portion of the anterior commissure is an exceedingly small fasci- 

 culus. It passes downwards and forwards, and finally enters the olfactory tract. 

 It is composed (1) of true commissural fibres, which bind one olfactory bulb to the 

 other ; and (2) of other fibres, which connect the olfactory bulb of one side with 

 the piriform area of the other side. 



The temporal portion is formed of almost the whole of the fibres of the 

 commissure. It is carried laterally under the lentiform nucleus, until it gains 

 the interval between the globus pallidus and the putamen. At this point it 

 changes its direction and sweeps backwards. In frontal sections through the 

 brain, posterior to this bend, the temporal portion of the anterior commissure 

 appears as an oval bundle of fibres cut transversely and placed in close contact 

 with the inferior surface of the lentiform nucleus (Fig. 576). Finally, it turns 

 sharply downwards on the lateral aspect of the amygdaloid nucleus, and its fibres 

 are lost in the white medullary centre of the temporal lobe. When the lateral 

 part of the anterior commissure is displayed by dissection, it is seen to be twisted 

 like a rope. 



The hippocampal commissure is composed of fibres which connect the hippo- 

 campus of one side with the corresponding structure of the opposite side. It is 

 described on p. 629. 



Association Fibres. The association fibres bind together different portions of 

 the cortex of the same hemisphere. They are grouped into long and short associa- 

 tion bundles. 



The greater number of the short association fibres pass between adjacent 

 gyri. They curve round the bottoms of the sulci in U-shaped loops. Some of 

 these occupy the deepest part of the gray cortex itself, and are termed intracortical 

 association fibres (Figs. 577 and 578) ; others lie immediately subjacent to the gray 

 matter between it and the general mass of the white matter and receive the 

 name of subcortical fibres. Many groups of short association fibres, instead of linking 

 together contiguous gyri, pass between gyri more or less remote. It is only 

 after birth, when intellectual effort and education have stimulated different portions 

 of the cortex to act in harmony and in conjunction with each other, that these 

 association fibres assume their sheaths of myelin and become functional. 



The long association fibres are arranged in bundles which run for considerable 

 distances within the white medullary centre of the cerebral hemisphere, and unite 



