796 THE SPINAL CORD. 



with an additional segment, e.g. the cat with fourteen ribs. This frequency of 

 individual variations exhibits in an almost ludicrous aspect the view held, that 

 each motor spinal root is a collection gathered together for a functional 

 purpose, and representing a particularly highly co-ordinated movement of the 

 limb. Contrary to any such hypothesis, it shows that for the functional 

 mechanism of the limb it is immaterial whether this or that particular group 

 of muscles is represented in a particular spinal root or not represented in it. 

 The absolute segmental level is variable over the range of nearly a whole 

 segment's length ; the relative segmental position is inviolably preserved. 

 Thus flexor carpi uhiaris may get fibres from as high as the sixth cervical 

 segment, or it may not, but it never extends so well forward, so far rostrally, 

 as flexor carpi radialis, and this latter never extends so far abo rally as flexor 

 carpi uhiaris, even when it is most post-fixed ; that is, the region of outflow 

 for the spinal nerve fibres to flexor carpi uhiaris lies rather more aborally than 

 that of the outflow to flexor carpi radialis, and this mutual segmental position 

 of the muscles is maintained whether the plexus be post-fixed or pre-iixed. 



In reflex movements, and in movements evoked from the cortex of the 

 brain, the discharge of motor impulses to a pluri-segmental muscle of the fore- 

 arm or wrist or hand, is probably always pluri-segmental, and involves the 

 whole length of the serial group of intraspinal nerve cells which innervates the 

 muscle extending through its full number of segments of the cord. The 

 afferent and cerebral channels of the cord treat the pluri-segmental motor 

 stations or nuclei of these limb-muscles as entities of homogeneous structure, 

 as, in fact, physiological units. 



This serves to emphasise the physiological homogeneity of limb-nmscle and 

 nerve trunk, and the physiological heterogeneity, in spite of morphological 

 unity, of the spinal nerve roots in the limb region of the body. The spinal 

 nerve roots of the thoracic region are, from the physiological point of view, less 

 heterogeneous. The peripheral nerve trunk is the physiological collection of 

 nerve fibres, e.g. flexors collected together, vaso-dilators included with motors 

 to muscles, etc. The nerve root is the morphological collection ; it contains, 

 commingled into one, such heterogeneities as adductors of the hallux with 

 protrusor muscles of the pelvic floor. 



Not all the efferent root cells are large. Many among the smaller intraspinal 

 cells probably innervate the vascular and visceral musculature. A numerous set 

 of such cells lies in a special lateral ridge of grey matter, extending in the cord 

 (man and monkey) from second thoracic segment to second lumbar inclusive. 

 This is the intermediate lateral cell-group lying in the lateral horn of spongiosa. 

 Gaskell first pointed out the correspondence between the longitudinal spinal 

 extent of this cell-group and the preponderance of minute myelinate fibres in 

 the efferent roots, and the existence in those roots of pressor paths to the 

 vascular musculature. The inference is, that the cell-group of the lateral horn 

 is composed not of mediate (or column) cells but of the root cells innervating 

 the musculature of the vessels (and viscera). The upper end of the group has 

 close topographical correspondence with the upper limit of outflow of nerve 

 fibres to the dilatator pupillae. The direct demonstration that a lateral horn- 

 cell gives an axon into the efferent spinal root has, however, not yet been 

 supplied. But it has been proved 1 that the minute nerve fibres passing by the 

 efferent roots into the sympathetic do come from cells lying very strictly in the 

 same segmental frontal level of the cord as the surface origin of the root 

 filament which contains them. They in this point resemble the relation of 

 the skeletal fibres to their cells of origin. 



It has been laid down as a law, that the muscles of a given part of the 

 limb and the sense organs in the overlying skin are supplied by the same 

 spinal segment, the afferent root cells being included as belonging to a spinal 



1 Sherrington, Journ. Physiol., Cambridge and London, 1892, vol. xiii. ; A. S. F. 

 Griinbaum, ibid., 1894, vol. xvi. 



