34 THOMPSON YATES LABORATORIES REPORT 
root of a thoracic nerve be carefully stimulated with gradually increasing strengths of stimulus, 
the musculature of the chest wall belonging to its own motor root is the first to reply, and all 
parts of it do not reply with equal readiness; certain portions are thrown into contraction more 
readily than others. The inter-costales are rather late ; as the intensity of the stimulus is 
increased, the effect, curiously enough, is not easily pushed across the median line ; long before 
that passes, some of the musculature supplied by the next adjacent motor roots behind and in 
front comes into play. It is difficult to note a distinct step in the spread, and the spread passes 
often a little earlier to the segment behind than to that in front.* Yet the intra-spinal resistance 
— to borrow electrical terminology — is for its own afferent path in the segment stimulated a little 
lower than in the adjoining. There is, therefore, some functional segmentation. But in the 
case of the limb muscles I could not obtain such evidence. Taking the flexion-adduction of the 
hallux of the monkey, this reflex could be obtained from each single one of a row of the rootlets 
of the three last roots of the limb-plexus. When obtained from an afferent rootlet of the first 
sacral root, it was obtained with equal facility whichever of the motor roots supplying the short 
muscles of the hallux remained unsevered. There was thus no evidence of a segmental barrier 
of resistance between the spinal segments concerned with the innervation of this muscle. These 
segments in regard to this muscle seem to have become so welded together as to form a physio- 
logical unit. 
There is, therefore, some evidence of functional segmentation in the cord ; but such 
evidence is yielded best by the Invertebrata. In the Invertebrata it is an ancestral heritage now 
so modified as to lie largely obscured from recognition. It is partly on the assumption of a 
functional segmentation of the spinal cord that the customary manner of dealing with the spinal 
functions as a separate and integral chapter of physiology is justifiable. Otherwise the contents 
of that chapter would more properly be distributed under the various special chapters devoted to 
the separate senses, and their organs and reactions. With Vertebrata this would, as a fact, for 
most purposes be the preferable plan. 
The individual is a mass of living units, their activity co-ordinated together by conductive 
strands (nerve-cells) reacting to the environment. The environment acts on this co-ordinating 
system through 'sense organs.' Of these there are sets each attuned to certain species of 
environmental changes. For some changes in the environment no sense-organs have been 
evolved, e.g., for 'Rontgen rays.' The channels of access for the environment are olfactory, 
visual, auditory, gustatory, cutaneous, and, finally, the muscular and viscera. We might imagine 
the form of the individual and the disposition of the sense-organs as primitively very simple; for 
instance, a spheroid with a digestive cavity and sense-organs diffused over the surfaces, especially 
the external. Such a form we should expect by evolution to become modified. Its contractile 
(muscular) mechanisms would obtain mechanical advantage (leverage) by elongation in certain 
directions. The lengthwise extension of the vertebrate body and of its lateral motor appendages 
the limbs, are in so far such as might be argued a priori. Such extensions involve simultaneous 
* Shenington, 'Phil. Trans. Roy. Soc.,' London, 1896. 
