302 PHYSIOLOGY CHAP. 



zone acquired its sensibility almost exclusively from the corre- 

 sponding dorsal root, while the remainder owed its sensibility both 

 to its proper root and to those adjacent to it. The cutaneous 

 root-zones or segments of the neck and trunk, according to Tiirck, 

 are arranged in series and girdle the body like rings, which start 

 from the spinous processes of the vertebrae and reach the ventral 

 median line in a direction almost vertical to the axis of the body. 

 The root areas for the skin of the limbs appeared to Tiirck to 

 be irregular in form, which in his day was found difficult to 

 interpret. 



Although commended by Ludwig in the second edition of his 

 Text-book, Tiirck's memoir passed almost unnoticed, the morpho- 

 logical theory of metamerism not being yet sufficiently developed. 



The modern view of the segmental distribution of the ventral 

 roots was led up to by the work of Ferrier and Yeo (1881) on the 

 motor roots of the brachial plexus in the monkey ; the almost 

 contemporaneous work of Paul Bert and Marcacci on the roots of 

 the lumbo-sacral plexus in the dog ; that of Forgue and Lannegrace 

 (1884) on the roots of the brachial and lumbo-sacral plexuses of 

 the dog and monkey; lastly, that of Polimanti (1894) on the 

 brachial and lumbo-sacral plexuses of the dog, rabbit, and cat. 

 The separate excitation of each of the ventral roots that combine 

 to form these plexuses invariably resulted in a synergic movement, 

 co-ordinated to a definite purpose, so that there is in the individual 

 ventral roots a functional systematisation of movements. 



The memoir of Forgue and Lannegrace is the most important 

 from the segmental point of view. These authors recognised that 

 each root contributes to the innervation of an always identical 

 series of muscles, so that in animals of the same species the 

 distribution is approximately constant. When a functional varia- 

 tion occurs it is small, and the innervation acquired or lost by 

 any root is borrowed from, or passed on to, the root immediately 

 adjacent to it, and not to a more distant root. In opposition to 

 the other authors cited, Forgue and Lannegrace assumed that 

 while the excitation of an entire root does produce a combined 

 movement, this combination is accidental and not functional, so 

 that normally, in carrying out any movement, the will must excite 

 the synergic fibres of several roots, and not of one root alone. 

 They showed no reason why this should be the case, but it 

 harmonises with the histological fact of the multiplicity of 

 collateral rami from the fibres of the pyramidal bundle, which 

 penetrate the grey matter at different levels and enter into relation 

 with the cells of the ventral horn in different segments. 



The theory of the metameric distribution of the sensory and 

 motor roots, now generally admitted, rests to a large extent upon 

 the exhaustive experiments of Sherrington (1893) on the sensory 

 roots, of Kisien Kussell on the motor roots of the monkey, and on 



