THE SPINAL ANIMAL 3S 
extension of the covering surfaces ; these surfaces are sentient, and the opportunity of these as 
sense-organs is almost in direct ratio as their mobility. The 'touch-field' is thus enlarged, and 
becomes enhanced with greater 'depth.' Perceptions of space are favoured. The tactual 
sensations reinforce and check more efficiently those based on the eyes, and on the semicircular 
canals. The visual sense owes its preponderance in providing perception of space doubtless to its 
stimuli emanating from enviromnental sources relatively remote. Its reactions thus come to 
usually precede in time the other reactions occurring between the organism and the environ- 
mental object. The brightness, or the darkness, or the colour affect the organism from a greater 
distance, and therefore, if the organism and object be relatively moving, in many cases earlier 
than do the other qualities of the object. Sensations of the eye in this way herald and forerun 
sensations that will in due time come to pass through other sense organs. Hence doubtless the 
almost constant relation between direction of vision and direction of locomotion ; the two 
coincide. Progression habitually takes the organism into that quarter of space already partly 
explored by his sense ; it carries him into his visual field. In the majority of cases the visual 
organs are so set that their field of view is occluded as little as possible by the extension of the 
organism itself. They are so set as to look 'out' from some projecting portion of the contour 
of the animal. When the form of the animal is elongate, as it so often is, mechanical advantage 
of leverage having been thus acquired, the visual organs approximate to one of the poles. That 
pole will then 'lead' in locomotion — the eyes look 'forward.' In other words, the motor 
mechanism that as it develops elongates, elongates primarily 'backwards.' And it is covered 
with skin possessing the usual cutaneous sense-organs. And it has its own 'muscular sense' due 
to organs embedded in itself. Side appendages (limbs) thrown off laterally from the elongate 
trunk repeat the primary or axial elongation as regards mechanical leverage for motor elements 
and the concomitance of increased sense apparatus of muscle and of increased sensorial skin 
surface. But, as a rule, the viscera do not extend into the lateral appendages, although for some 
distance into the primary axial or vertebrate, thus ensuring increased absorbent surface. Hence 
there comes to be a great motor apparatus extending back behind the visual organ, and containing, 
besides muscle, a certain amount of visceral cavity; and covered with skin containing cutaneous 
sense-organs. This apparatus it is which chiefly executes the movements of the organism as a 
whole. It can alter the space relations of the mouth and sense-organs in regard to the environ- 
ment, and thus facilitate the nutrition of the organism by securing food; it can remove the 
individual from situations of danger or of injury. It is at the command not only of the sense- 
organs resident in itself and in its own covering, but also of the great sense-organs near that pole 
of the animal which its function is to drive 'forward' into visual space. The pole where the 
great projicient senses of sight, smell, taste, liearing, and stereotropism (semicircular canals) have 
their organs, is the pole that, by the action of the motor train attached, 'leads' in progression, and 
the motor organ itself is after all in the main tlieir instrument. As the animal scale is ascended 
it becomes their instrument more and more. The pole at which they lie is called by anatomists 
'the head,' and the characters which the great sense-organs and apparatus for intake of food 
impress upon it make its identification easy throughout a vast range of animal form. It is signifi- 
