THE TEGMENTUM OF THE CRUS CEREBRI AND ITS GANGLIA. 421 



and widest tract is common to the caudate and lenticular 

 nuclei. The substance of Soemmering participates merely in 

 the formation of the middle and internal area proceeding from 

 the ganglia. 



The sizes of the cerebral lobes and of the crusta of the cerebral 

 peduncle, with its ganglia, rise and fall together in the brains of dif- 

 ferent Mammals, proportionally to one another. If, for example, the 

 cerebral lobes in Man, the Baboons, and the Roe-deer amount respect- 

 ively to 78, 70'8, and 62 per cent, of the whole brain, the ganglia of 

 the crus (insula, with the corpus striatum internum and externum) 

 form in the same series 58, 40, and 33-3 per cent, of all the struc- 

 tures composing the cerebral peduncle, and the size of the crusta to 

 that of the tegmentum in Man is as 1 : 1 ; in the Baboons as 1 : 3 ; 

 and in the Roe-deer, as 1:5. Moreover the preponderance in the 

 case of tbe lenticular nucleus in Man is far larger than in the case of 

 the nucleus caudatus, which is dependent upon the circumstance that 

 the latter is only continuous with what is in Man an atrophied portion 

 of the hemispheres the olfactory lobes. 



THE TEGMENTUM AND ITS GANGLIA. 



The variations presented by the ganglionic region of the 

 posterior tract of origin of the spinal cord show that its size 

 is independent of the development of the lobes of the cerebrum. 

 In the brain of Man, indeed, the relative size of this region 

 is remarkably diminished in comparison with that of Mam- 

 mals. For example, in Man the optic thalami form 19 per cent. ; 

 in Monkeys, 30 per cent. ; in the Roe-deer, 22'9 per cent, of the 

 entire mass of the sensory ganglia : whilst the corpora quadri- 

 gemina in Man amount to 6*5 per cent.; in Monkeys, to 10 

 per cent. ; and in the Roe-deer, to 16*6 per cent, of the same 

 group of organs. 



The chief ganglia of origin of the tegmentum are (1) the 

 optic thalami and (2) the corpora quadrigemina. Besides the 

 common feature of serving as centres of origin for the spinal 

 cord, these ganglia are also alike in their connection with the 

 tractus opticus. The latter feature they share with the cor- 

 pora geniculata, which must doubtless be regarded as an 

 appendage to the above-named ganglionic system. Further 



