INTERNAL STRUCTURE. ."65 



sense and motion on the other, be regarded as a whole, it is seen to traverse the 

 second and third groups of grey substance, that of the cerebral ganglia, and that 

 of the central grey substance. These divide it into three segments, — an upper, 

 middle, and lower. These three segments are termed j»-oJrcti/»i .^//.sfeiiis, since 

 the function of the whole nervous tract may be considered as being to project 

 the external world on the cerebral convolutions, and conversely the changes in 

 the cerebral convolutions upon the motor organs. 



Fir^t 2>^'ojectio)i systcni (P. S. I.) between the convolutions above and the 

 cerebral ganglia (corpus striatum, &c.) below, corresponds for the most part to 

 the corona radiata (Stabkranz). 



Second 2)rojcction sijstcm (P. S. II.), between the cerebral ganglia above and the 

 central grey matter below. As the latter extends from the fifth ventricle to 

 the lower end of the spinal cord, the fibres of this system are of very various 

 length'. 



Third projection sijsteni (P. S. III.) from the grey matter of the central cavities 

 to the muscles and tei-minations of the sensory nerves, corresponding nearly to 

 the peripheral nerves. 



The other systems of fibres are (as commonly enumerated) the commissural, 

 consisting of the corpus callosum and anterior commissure, which unite identical 

 regions of diflferent hemispheres, and the association system of fibres which unite 

 non-identical regions of the same hemisphere. 



In the passage from P. S. I. to P. S. II., which is effected in the cerebral 

 ganglia, the fibres undergo considerable reduction in number, the fibres of the 

 crus being much fewer than those of the corona radiata. On the other hand, in 

 the transition from P. S. II. to P. S. III., in the grey substance of the central 

 cavities, the fibres undergo a great increase in number : the peripheral nerve- 

 fibres being much more numerous than those of the crus or cord. 



The division of the fibres of the crus into two portions, an anterior or lower 

 criisfa (Fuss), and an upper or posterior tegmentum (Haube), maj- be extended 

 to the cerebral ganglia in which the fibres of each portion respectively terminate 

 abjve. 



The ganglia connected with the crusta are the lenticular nucleus, the coi-pus 

 striatmn proper, and locus niger. They are connected (by P. S. I.) chiefly ^vith 

 the anterior part of the brain, and subserve chiefly voluntary motion. 



The ganglia of the tegmentum are the optic thalami, corpora quadrigemina, 

 corpora geniculata interna, coi-pora albicantia, and subserve chiefly reflex 

 movements. 



Certain fibres, thought to be sensory, pass up from the posterior columns of 

 the cord, and form the posterior and outer fasciculi of the crusta of the crus 

 cerebri. They pass tlirough no ganglion, but ascend behind the optic thalamus 

 to the cortex of the temporal lobes. 



The cerebellar grey substance is connected with that of the cerebrum by two 

 groups of fibres. 



1. The connecting arm (Bindeann), processus a cerebello ad cerebriun, arises 

 from the corona radiata, and passes under the thalamus and corpora quadri- 

 gemina, to mix with the fibres of the tegmentum, and reach the cerebellmu 

 after a total decussation. 



2. The size of the crusta above the pons, is much greater than that of the 

 motor tract below ; and this is due to the fact that some of its fibres turn aside 

 in the pons and reach the cerebellum through the middle peduncle. 



The union of the cerebellum with the spinal cord is also double, through the 

 fasciculus cuneatus and fasciculus gi-acihs, from the posterior column, and the 

 restiform body, fi'om the lateral column. 



The large size of the pons is due to the interlacement of the arms of the cere- 

 bellum with the projection system. 



ORIGIN OF THE CEANIAIi NERVES. 



The smface attachments of the cranial nerves remain to be described, 

 and it will be couveaieut to recapitulate at the same time their deep 



connections. 



