FUNCTIONS OF THE ANTERIOR SPINAL ROOTS. 



619 



A 



\ 



Inco-ordinated Movements of Insensible Limbs. After section of the posterior roots, e.g., 

 of the nerves for the posterior extremities, the muscles retain their movements, nevertheless 

 there are characteristic disturbances of their motor power. This is expressed in the awkward 

 manner in which the animal executes its movement it has lost to a large extent its harmony 

 and elegance of motion. This is due to the fact 

 that, owing to the absence of the sensibility of the 

 muscles and skin, the animal is no longer conscious 

 of the resistance which is opposed to its movements. 

 Hence, the degree of muscular energy necessary for 

 any particular effort cannot be accurately gradu- 

 ated. Animals which have lost the sensibility of 

 their extremities often allow their limbs to lie in 

 abnormal positions, such as a healthy animal would 

 not tolerate. In man also, when the peripheral 

 ends of the cutaneous nerves are degenerated, there 

 are ataxic phenomena ( 364, 3). 



Increased Excitability. Harless (1858), Ludwig, 

 and Cyon (controverted by v. Bezold, Uspensky, 

 Griinhagen, and G. Heidenhain) observed that the 

 anterior root is more excitable as long as the pos- 

 terior roots remain intact and are sensitive, and 

 that their excitability is diminished as soon as the 

 posterior roots are divided. In order to explain 

 this phenomenon, we must assume that, in the 

 intact body, a series of gentle impulses (impres- 

 sions of touch, temperature, position of limbs, &c.) 

 are continuously streaming through the posterior 

 roots to the spinal cord, where they are transferred 

 to the motor roots, so that a less stimulus is required 

 to excite the anterior roots than when these reflex 

 impulses of the posterior root, which increase the 

 excitability, are absent. Clearly, a less stimulus 

 will be required to excite a nerve already in a gentle 

 state of excitement than in the case of a fibre which 

 is not so excited. In the former case, the dis- 

 charging stimulus becomes as it were superposed on 

 the excitement already present. (Compare 362.) 



The anterior roots of the spinal nerves 

 supply efferent fibres to 



1. All the voluntary muscles of the trunk 

 and extremities. 



Every muscle always receives its motor fibres from 

 several anterior roots (not from a single nerve-root). 

 Hence, every root supplies branches to a particular 

 group of muscles (Preyer, P. Bert, Gad). The ex- 

 periments of Ferrier and Yeo show that stimulation 

 of each of the anterior roots in apes (brachial and 

 lumbo-sacral plexuses) caused a complex co-ordi- 

 nated movement. Section of one root did not cause 

 complete paralysis of the muscles concerned in 

 these co-ordinated movements, although the force 

 of the movement was impaired. These experiments 

 confirm the results of clinical observation on man. 

 The fibres for groups of muscles of different func- 

 tions {e.g., for flexors, extensors) arise from special 

 limited areas of the spinal cord. The cervical and 

 lumbar enlargements of the spinal cord are great 

 centres for highly co-ordinated muscular move- 

 ments. 



2. The anterior roots also supply motor 

 fibres for a number of organs provided with smooth muscular fibres, e.g., the bladder 

 ( 280), ureter, uterus. [These are the viscero-motor nerves of Gaskell, and from 

 them come also viscero-inhibitory nerves.] 



3. Motor fibres for the smooth muscular fibres of the blood-vessels, the vaso- 



Fig. 438. 

 Distribution of the cutaneous nerves of the 

 leg (after Henle). A, Anterior surface 

 1, crural nerve ; 2, external lateral 

 cutaneous ; 3, ilio-inguinal ; 4, lumbo- 

 inguinal ; 5, external spermatic ; 6, pos- 

 terior cutaneous ; 7, obturator ; 8, great 

 saphenous ; 9, communicating peroneal ; 

 10, superficial peroneal; 11, deep peroneal; 

 12, communicating tibial. B, Posterior 

 surface 1, posterior cutaneous ; 2, ex- 

 ternal femoral cutaneous ; 3, obturator ; 



4, median posterior femoral cutaneous ; 



5, communicating peroneal ; 6, great 

 saphenous ; 7, communicating tibial ; 8, 

 plantar cutaneous ; 9, median plantar ; 

 10, lateral plantar. 



