90S THE CENTRAL NERVOUS SYSTEM 



which the scratch- reflex is discharged (in the dog), are freely con- 

 nected with efferent paths to the muscles of the hip, knee, and ankle 

 by an uncrossed path descending the lateral column (Sherrington), 

 In cats, after temporary occlusion of the cerebral circulation, which 

 throws the brain out of gear, it is easy to elicit movements of the 

 hind-legs by pinching the fore-paws or the skin of the 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 combina- 

 tion of muscular actions for the production of appropriate and har- 

 monious 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 recognize 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. These reactions are necessarily purposive in 

 character, since they have been evolved with reference to the ad- 

 vantage of the organism as a whole. They are the sort of complex 

 reactions which the intact animal would have had to improvise 

 by the combination of a considerable number of simple movements 

 when it was executing such defensive reactions, with the conscious 

 purpose of escaping from the irritant, were they not already present 

 as purposive reflexes in the ready-made condition. 



In the combining of reflexes we may distinguish between simul- 

 taneous combination i.e., the combination of reflex actions taking 

 place at the same time and successive combination 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 indis- 

 criminate, but follows a definite ' march,' determined partly by the 



