248 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Interpreting these facts in the terms 

 of nerve-cells and their arrangement, 

 it appears that in the shoulder-centre 

 the axones of the cortical cells that 

 discharge downward affect predomi- 

 nantly the efferent cell-groups which 

 in the spinal cord directly control the 

 muscles of the shoulder, and that a 

 similar arrangement obtains for the 

 other centres in this region with the 

 corresponding cell-groups in the cord. 

 The stimulation of the different por- 

 tions of the internal capsule where it 

 is composed of bundles of fibres coin- 

 ing from the motor region shows (ob- 

 servations on orang-utang) that the 

 fibres running to the several lower 

 centres are there aggregated and 

 arranged in the same order as the 

 cortical centres from which they arise 

 (see Fig. 102). 



Separateness of Areas and Cen- 

 tres. — As we ascend in the mammalian 

 series there is an increase in the per- 

 fection with which cells forming the 

 several centres are segregated, though 

 the areas in the different orders tend 

 to hold the same relative positions. 1 



Figs. 103, 104 give the localizations 

 obtained in the rabbit's brain by stim- 

 ulation (Mann). The various areas 

 occupy a large proportion of the cortex, 

 , and in some cases come very close to- 



Fig. 102.— Horizontal section of the human cere- J 



brum, showing the internal capsule on the left gethcr, SO that thev are not easily Sepa- 

 side: /', frontal region: G, knee of the capsule ; , , . ' 



NC.NC, muriate nucleus; NL, lenticular nucleus; rated by experiment. 



0, occipital lobe; TO, thalamus; X,X, lateral ven- J n the lower monkeys (Macacvs 



trir-lc In the internal capsule the letters indicate 



the probable position of the bundles of tii.r.-s whir h Milieux) these cell-groups are SCgre- 



upon stimulation give rise to movements of the w^l g0 <),.,, those associated with the 



l>urts named or which convey special sets of in- © > _ i i » 



coming impulses: E, eyes ; ff.head; r.tongue; M, cervical portion Of the cord and lomi- 



mouth : L, shoulder; B.elboy ; D, digits; .A.abdo- ,i __ __„_i, .„..„„,, *■„ 



, ' ,. , ... . . mtr the arm-area are nmen more to- 



men ; P.hip; if, knee; U, toes; S, temporo-occip- a 



itai tract ; oc, fibres jo the occipital lobe ; op, optic gether and quite separate from those 



radiation (based on Ho rel . , ■, ..i ,1 1 i • ,1 



associated with the lumbar region, the 

 leg-area. In the orang-utang,- and to a greater extent in man, a further sepa- 



1 Mann : Journal of Anatomy and Physiology, 1895, vol. xxx. 



2 Beevor and Horsley : Proceedings of the Royal Society, London, 1890-91, vol. xlviii. 



