476 SIR CHARLES BELL ON THE BRAIN. 



and medulla oblongata detached from the great masses of the cerebrum and cerebel- 

 lum, so that they may lie before us. We should first mark out and trace the columns 

 of the spinal marrow ; observing the corpora restiformia as they come down from 

 the cerebellum, we may split them at the posterior fissure and fold them aside. 



We now survey the extent of the fourth ventricle. On each side of the calamus 

 scriptorius are two pyramidal columns*. To trace these upwards we must cut into 

 the iter ad tertiam ventriculam, by dividing the corpora quadrigemina, and then we 

 can trace them up into the thalami nervorum opticorum. By a section we may 

 trace them through that body, and then diverging into the hemispheres of the cere- 

 brum. 



Having followed these columns upwards, we next trace them downwards, and find 

 that they join, intermingle, and decussate, and again separate, and proceed down the 

 spinal marrow (Plate XX. fig. 2. b. c). 



From no part of this column does any nerve of motion take its origin ; its relations 

 to the sensitive nerves will be seen on further dissection. 



The corpus striatum and the thalamus lie very curiously together ; the thalamus 

 forms a nucleus round which the corpus striatum bends, and when their respective 

 layers of striae make their exit beyond these bodies to form the great fan- or solar- 

 like expansion into the hemisphere of the cerebrum, their rays mingle together. A 

 rude representation of these two tracts of the cerebrum, as we have traced them, 

 may be made with the hands. If I place my wrists together, parallel, and closing 

 one hand, embrace it with the other, I represent the two portions of one crus. The 

 closed fist is the thalamus, and the other is the corpus striatum. If I then extend my 

 fingers, interlacing their points, I represent the final distribution of the portions of 

 the nervous matter which are dedicated to sensation and volition. 



But before proceeding further, we must distinguish a certain portion of the great 

 tract of fibrous matter that lies behind the septum of the pons, which does not belong 

 to sensibility, but to a different order of parts. If we dissect round the corpus olivare, 

 we find it easy to separate this body from the column of motion on the fore part, and 

 the column of sensation behind. Following then the fibrous portion of matter which 

 ascends from it, we find that it runs close upon the back of the septum of the pons, 

 and that a part of it goes ofi* to the corpora quadrigemina, whilst a part runs directly 

 into the crus cerebri. 



On tracing the column which descends from the corpus olivare, we find that it is 

 very soon attached to the columns both of motion and of sensation, and becomes in- 

 corporated with them as it passes downwards (Plate XXI. g.). 



We have now traced three great tracts or courses of fibres into the crus cerebri ; 



* In fact all the columns which form the medulla oblongata converge downwards and are pyramidal. We 

 have the anterior pyramidal bodies, the posterior pyramidal bodies or corpora restiformia, and those deeper 

 columns, whose form might authorize the term, as they are more especially counterparts of the true anterior 

 pyramidal bodies. 



