36 THOMPSON YATES LABORATORIES REPORT 
cant in the evolution of animal form that the organ that exhibits most uninterrupted and 
harmonious increase in development as studied successively in passing from lowest to highest is the 
brain. And it is significant that in the nervous system — segmental system as it is — the brain is 
developed not in those segments whose sense-organs are ordinary cutaneous (tactual, &c.), 
muscular and visceral, but in the segments connected witli the visual, olfactory, and otic sense 
organs; in other words, the brain is developed in the 'head.' The head is, so to say, the 
individual ; it has the mouth, it takes the food, including air and water, and it has the main sense 
organs providing the data for both space and time. To this the body, an elongated motor organ, 
with a share of the viscera, and the skin, is appended primarily as a machine for locomotion. This 
latter must of necessity lie at behest of the great sense-organs of the head. 
Hence there are nervous conductors from the great sense-organs of the head to the great 
motor organ which the contractile masses of the body together form. The spinal cord contains 
this strand of conductors, and it is in tliis sense a mere appendage of the brain. But the motor 
organ itself is a complex structure, built up of many parts. These, if the movements asked for 
by the projecting senses are to be adequately executed, must be co-ordinate in action. One of 
the main functions of the spinal cord as an independent organ, and apart entirely from the 
influence of the cranial sense-organs, is the regulation of the activities of the several muscles, so 
that they act co-ordinately. This is well illustrated by analysis of the muscular movements 
which ensue in the limbs as result of reflex excitation in the purely spinal animal. In the limb 
region, in response to an excitation of a single afferent root, the spinal discharge of centrifugal 
impulses evoked tends to occur by more than one efferent root.* It is a plurisegmental 
discharge to plurisegmental muscles. But 
though plurisegmental it is in each segment 
only fractional. It treats any one muscle 
of the limb as an entity, either exciting it 
wholly or leaving it wholly alone. The 
contraction of the various segmental por- 
tions of the plurisegmental muscles are 
thus co-ordinated together. They are knit 
together by a co-ordination which is 
wholly spinal. Further, the co-ordination 
of antagonistic muscles is managed by 
the spinal cord. After Winslowt and 
DuchenneJ it became current doctrine that 
antagonistic muscles act co-operatively in 
the sense that the antagonist, by contracting 
synchronously with the prime muscle, 
* Sherrington, 'Journ. of Physiol.,' xiii, 1892. f 'Anatomical Expositions,' &c., 1749. 
X 'Localisation,' &c., 1867, and Rieger, 'Archiv f. Psych.,' xiii. 
