THE CENTRAL NERVOUS SYSTEM 805 



upper part of the body. The scratch-reflex can also be very 

 readily evoked, and in great intensity, by stimulating the pinna, 

 and is not confined to the side stimulated. In anaemia of the brain 

 and (cervical) cord and subsequent resuscitation, homolateral 

 reflexes (i.e., on the same side as the stimulus) are submerged later 

 and emerge sooner than contralateral reflexes whose centres lie in 

 the area which was rendered anaemic (Pike, Guthrie, and Stewart). 



Co-ordination of Reflexes. The co-ordination or orderly 

 combination of muscular actions for the production of appropriate 

 and harmonious movements is one of the most important 

 functions of the central nervous system. Both the brain and the 

 cord take a share in this co-ordination. The role of the brain 

 will be considered later on, but it is essential to recognise now 

 that many of the movements which the brain directs represent 

 spinal reflexes already synthesized, compounded, or co-ordinated 

 in a very high degree. This is the reason why, in the spinal 

 animal, the inexperienced observer may sometimes be startled 

 by the apparently ' purposive character ' of a reflex movement 

 the scratch-reflex in the dog or cat, e.g., or the extensive reflex 

 movements of the hind-legs of a brainless frog when the skin is 

 pinched or painted with dilute acid, so plainly directed to the seat 

 of irritation. When a drop of acid is applied to the flank of such a 

 frog, it will attempt to wipe it off with the foot which is situated 

 most conveniently. If this foot be held, it will use the other. 



In the combining of reflexes we may distinguish between 

 simultaneous combination i.e., the combination of reflex 

 actions taking place at the same time and successive com- 

 bination i.e., the combination of reflexes in such a way that 

 they follow each other in an orderly sequence. The facts already 

 mentioned in speaking of irradiation afford a partial explanation 

 of the co-ordination of reflexes by simultaneous combination. 

 The movements are orderly and harmonious because the spread 

 of the reflexes is not indiscriminate, but follows a definite 

 ' march,' determined partly by the anatomical relations of afferent 

 and efferent paths, partly by the varying resistance of the 

 synapses or other structures whose properties fix the threshold 

 value of the excitation by which an arc can be forced. In 

 general it is not enough that the channel of the final common 

 paths (p. 798) to the muscles whose contraction produces the 

 reflex movement should be thus open to the afferent arcs that 

 elicit the movement ; they must be closed to other afferent arcs 

 which might disturb the reflex. Not only so : there is evidence 

 that very frequently the final common paths are, so to say, more 

 widely opened to the afferent arcs in question by the ' reinforcing ' 

 or ' facilitating ' influence of allied, though it may be distant, 

 afferent arcs, which are simultaneously excited (p. 807) . Further, 



