THE WAY OUT. 



795 



muscle alone and apart from its fellows. Even the severance of a whole single 

 motor nerve root cannot paralyse a single limb-muscle ; the effect of such an 

 injury is to partially impair a large number of the muscles. 



The individual extrinsic eye muscles innervated by the oculo-motorius have 

 been assigned to certain groups of cells in the nucleus of that nerve. The 

 evidence is, however, conflicting. A demonstrably segmental grouping of the 

 efferent root cells in the human spinal cord has been assumed by some anat- 

 omists ; l the statement rests on some evidence of such a grouping in the eel 2 and 

 snake. 3 Waldeyer assumes a similar arrangement in the cord of the gorilla. 

 The careful examinations of the mammalian (including human) cord by Kaiser 4 

 and v. Argutinski 5 have shown that no segmental 6 grouping of any of the 

 cell columns is detectible; this conclusion stands in harmony with the 

 results of the examination of the relation of the reflex movements to spinal 

 segments. The results of an analysis of the spinal nerve supply of the 

 muscles of the limbs show that the muscular tissue of the limb is arranged in 

 a number of rays, a ray for each metamere contributing to the limb. It 

 shows that of these rays the ones which lie tailmost in the fore-and-aft series 

 are the longest, that is, extend to the extreme free apex of the limb, whereas 

 the foremost, the most rostral, pass only, in the fore-limb, as far as the upper 

 arm, the next only as far as the elbow, the next only as far as the wrist. The 

 four last, or most aboral rays, all contribute to the musculature of the hand in 

 the common macaque monkey (Macacus rhesus}. 



When we come to inquire how these units of the segmental architecture 

 of the limbs, these muscular rays, are related to the physiological or functional 

 units of the limb musculature, it is at once obvious that the extent and 

 boundaries of the two do not coincide. The individual muscles of the limb 

 are functional elements of its structure as a physiological machine. But each 

 of these functional elements is compounded of portions of several rays or 

 myotoms. Moreover, the boundaries between the myotoms do not correspond 

 with the intervals of muscles, or even between muscle-groups. 



The same want of segmental separation is evident when we examine the 

 location of the nerve cells which innervate the individual muscles. In 

 experiments on the plexuses, it is rare to find two consecutive individuals 

 of the same species that possess a similar root distribution. The plexus 

 in any species requires for description to be grouped into classes. For most 

 purposes it suffices to distinguish a class in which the plexus is pre-fixed 

 and a class in which it is post-fixed. Thus, in some individuals, the supinator 

 b re vis is innervated from the sixth and fifth cervical nerves, in others from 

 the sixth and seventh. In the former case the plexus is of pre-fixed type. 

 When one muscle or one part of the plexus is pre-fixed, all the rest are pre- 

 fixed ; and conversely, when one part is post-fixed, all are post-fixed. In the 

 second instance given above, the supinator brevis is post-fixed, that is, its 

 nerve cells are fixed in segments further aboral in the series than in the 

 individuals with pre-fixed plexuses. Presumably, the muscle itself therefore is 

 built of myotoms more aboral in the series than in the muscle in the pre-fixed 

 individuals. Between the extreme cases of post-fixed type and the extreme 

 examples of pre-fixed type extend a numerous series of intermediate individuals; 

 in fact, any and every intermediate degree of type seems to exist. The 

 extreme of post-fixed type passes over into the comparatively rare individual 



1 Schwalbe, "Lehrbuch d. Neurologic, " in Hoffmann's "Lehrbuch der Anat, d. 

 Menschen;" Waldeyer, " Das Gorilla-nick enmark," Berlin, 1889. 



2 Stieda, Ztschr. f. Wissensch. Zool., Leipzig, 1868, Bd. xviii. 

 3 Luderitz, Arch. f. PhysioL, Leipzig, 1881. 



4 "Die Funktionen d. Ganglienzell. d. Halsmarkes," Haag, 1891. 



5 Arch.f. mikr. Anat., Bonn, 1896, Bd. xlviii. 



6 The cells of the columns, especially of that of the lateral horn, are collected into groups 

 along the length of the cord, but the groups are much more numerous than are the 

 segmental nerves. 



