794 



THE SPINAL CORD. 



ment, the closure of the hand or foot, is, as a volitional movement, lost. The 

 nutrition of this group seems therefore to be especially dependent on the main- 

 tenance of the local reflex arc (and perhaps reflex tonus). It is said 1 that the 

 reflex collaterals are more numerously traceable to this part of the ventral 

 horn than to others. The efferent root cells innervating any one muscle do 

 not seem to be collected into a definite group in the frontal planes of the 

 cord. In the limb regions the root cells innervating each single muscle extend 

 throughout the length of two, three, or more segments of the cord. 



In the higher vertebrates each of the segmental spinal nerves which 

 supplies the musculature of the limb leaves the spinal cord in the form of a 

 row of rootlets, which together build up the motor root of the nerve. Excita- 

 tion by electric currents of any one of these rootlets or filaments, that contribute 

 to the formation of the motor root, produces a movement in the limb which 



resembles closely the 



c.l 



movement produced 

 by excitation of the 

 whole root. The same 

 muscles are thrown 

 into contraction as 

 when the whole root 

 is excited ; but the 

 action of each muscle 

 is feebler. The col- 

 lection of nerve fibres 

 contained in the root- 

 lets is therefore, in 

 miniature, a collection 

 like that contained in 

 the whole root. This 

 fact must be taken 

 together with the fact 

 that, as can be shown 

 by degeneration ex- 

 periments, the fibres 

 contained in each root- 

 let represent the axis- 

 cylinders of the motor 

 nerve cells lying in 

 that particular trans- 

 verse plane of the cord, 

 from which the rootlet 

 makes its emergence. 

 The two observations 



Fig. 355. — Diagram showing arrangement of the root fila- 

 ments of two spinal nerves, c.a. The ventral white 

 column of the cord ; c.l. the lateral white column ; 

 c.p. the dorsal white column ; s.m.l.a. the median 

 ventral fissure of the cord ; r.a. the ventral root ; r.}). 

 the dorsal root ; g.g. the ganglion of the dorsal root. — 

 From van Gehuchten. 



together show that nerve cells which innervate the different muscles belongin^ 

 to any one myotom in the limb lie commingled, so that any one section 

 through the spinal cord at right angles to its length will meet nerve cells 

 belonging to all the muscles represented in that segment. This is the 

 same as saying that the nerve cells belonging to any one limb-muscle lie 

 not gathered together at any one particular level of the spinal segment, 

 but scattered throughout its length. And inasmuch as each muscle is in- 

 nervated from several segments, the nerve cells for each muscle are scattered 

 in a continuous series through the length of a series of spinal segments ; and 

 throughout this extent are commingled with the cells of a great number— in 

 some cases as many as forty and more— of other muscles. It is therefore 

 evident that no traumatic injury of the spinal cord can ever paralyse a single 



'O. Mingazzini, Riv. sper. di freniat., Reggio-Emilia, 1892, tome xviii. Nos. 3-4; 

 Lugaro, Rw. dipatol. nerv., Firenze, 1896, tome i. p. 1. 



