242 SPECIAL PHYSIOLOGY. 



and form almost infinite communications amongst themselves, and with 

 the nerve cells. 



The internal substance, a, a, of the hemispheres, named the medul- 

 lary substance, is white, and is chiefly composed of white nerve fibres 

 of very small diameter, which are ultimately connected with those 

 found in the cortical substance. They may be regarded as forming 

 several systems. First, there is a system of short intrinsic commissu- 

 ral fibres immediately beneath the gray matter, which connect adja- 

 cent or even remote convolutions. Secondly, there exists another 

 smaller but doubtless important system of intrinsic longitudinal com- 

 missural fibres, which serve to connect the several lobes of each side, 

 and also the so-called optic thalami and other parts, with the occipital 

 and temporal lobes. The chief longitudinal band constitutes a part 

 called the fornix, situated in the ventricular cavities beneath the cor- 

 pus callosum ; besides this, there are the tcenia semicircularis, also 

 within the ventricles, and certain other bands above the corpus callo- 

 sum, and within the convolutions resting immediately upon it. Thirdly, 

 there is a system of transverse commissural fibres, which pass from all 

 regions of one hemisphere to the corresponding regions of the other, 

 and form the large cross band called the corpus callosum, Fig. 59, c, 

 above the ventricles ; three other small transverse commissures, ante- 

 rior^ middle, and posterior, are situated in the floor of the ventricles. 

 Lastly, there are found fibres, usually named the radiating fibres, or 

 ascending fibres, but which, if traced from the cortical substance of 

 the hemispheres downwards, may be called the convergent or descend- 

 ing fibres. These proceed from all parts of the superficial gray matter 

 of the hemispheres, and converge towards the cerebral peduncles. 

 Some of these convergent fibres have been described as crossing from 

 one hemisphere, through the corpus callosum, to the other, and then 

 descending towards the cerebral peduncles of that side, constituting 

 therefore a decussating system of fibres ; but this is not certain. 



Above the cerebral peduncles are found, on each side, two large 

 masses of gray matter, named the corpus striatum and the optic thala- 

 mus. The corpora striata lie in front of the optic thalami; both 

 bodies partly project into the ventricular cavities, partly rest upon the 

 peduncles, and have the rest of their surface embedded in the corre- 

 sponding hemisphere, the radiating or convergent fibres of which, com- 

 ing from all directions, pass into one or both of these masses of gray 

 matter, but especially into the corpus striatum, which hence presents 

 a streaked appearance on a section, as its name implies. It was for- 

 merly supposed that the convergent fibres from the hemispheres, trav- 

 ersed these large masses of ganglionic gray matter, and became di- 

 rectly continuous with fibres in the cerebral peduncles, the pons, the 

 medulla oblongata, and the spinal cord. But, according to the most 

 recent view, the fibres proceeding downwards from the cortical sub- 

 stance of the hemispheres, do not pass continuously through the cor- 

 pora striata and optic thalami, into the cerebral peduncles, but termi- 

 nate in the gray substance of those ganglionic masses, from which other 

 fibres pass down into the peduncles, medulla oblongata, and spinal 

 cord, and so become connected with the roots of the cranial and spinal 



