DIRECTION OF IMPULSES IN REFLEX ACTIONS. 82 



j 



My own experience in the mammal is far from completely accordant with 

 this statement ; in, I think, the majority of instances, irradiation has spread 

 more easily down than up the cord. It is easy to obtain reflex move- 

 ments of the limbs and tail by excitation of the skin of the pinna, whereas the 

 reverse is rare. To elicit by excitation of the hind-limb a movement of the 

 fore-limb, is more difficult than by excitation of the fore-limb to elicit move- 

 ment of the hind-limb. To elicit movement of the tail by excitation of the 

 fore-limb, is easier than to move the fore-limb by excitation of the tail. The 

 irradiation has in my experience been easier across the cord from hind-limb to 

 hind-limb, than from hind-limb to fore-limb ; but it is often easier down the 

 cord from fore-limb to hind-limb, than across from fore-limb to fore-limb. In 

 such reflexes also as the " shake " reflex (see p. 815), which implicate the trunk 

 more than the limbs, the radiation is away from the head, for it is well ob- 

 tained as a rump reflex when the skin of the shoulder is the part rubbed. 

 In the " scratching " reflex, too, the skin stimulus is applied far headward of 

 the region of the muscular contraction evoked. 



- D-* 



These so-called " laws " of reflex irradiation have been so generally 

 accepted as to have obtained a doctrinal importance which they really 

 hardly merit. It seems here less profitable to attempt adapting Pfmger's 

 " laws " so as to make them better fit the observed facts, than to 

 briefly describe the salient features of the long intraspinal reflexes 

 as exhibited in an ordinary experiment on the spinal mammal. 



When the animal is supported freely from above with its spinal axis 

 horizontal, and the attitude of the limbs determined by gravitation, a 

 point that early strikes the observer is that there are eight areas of skin 

 whence spinal reflexes employing skeletal musculature can be provoked with 

 pre-eminent facility. These areas are the soles, the palms, the pinnae, and the 

 tail and cloacal region. It is significant that seven of these areas are those 

 which possess the greatest range of motility if the axis of the animal be 

 considered fixed. From the pinna may be excited movements of all the limbs, 

 the neck, the tail, and trunk. The irradiation from this reflexigenous area 

 usually presents the following order — (1) Neck and homonymous fore-limb, 

 (2) homonymous hind-limb, (3) tail and trunk on both sides, (4) contra- 

 lateral hind-limb, (5) contralateral fore-limb. From the fore-foot can be 

 excited, besides movements in the fore-limb itself, movements in the other 

 limbs and tail. The facility of irradiation is usually in the following descend- 

 ing series — (1) homonymous hind-limb and the tail, (2) crossed hind-limb, (3) 

 crossed fore-limb. The relative facility of spread of the reaction to the crossed 

 fore-limb seems subject to much variation. In the frog the path between the 

 two fore-limbs is, especially in the breeding season, very open and facile. In 

 the cat and monkey, it seems to be much more open in the bulbo-spinal 

 than in the spinal animal. From the hind-foot the frequency and ease of 

 irradiation into other spinal regions usually appears to exist in the following 

 order — (1) crossed hind-limb and tail, (2) homonymous fore-limb, (3) 

 crossed foredimb. The facility of "spread" from one lateral half of the cord 

 to the other is very dissimilar at different levels of the cord. It is particularly 

 easy in certain parts of the tail region, and these motor mechanisms which are 

 yoked together are for the most part, as with the flexion and extension of the 

 hip and knee, of an asymmetrical kind. In the hind-limb region the crossed 

 irradiation is also fairly free, and largely connects asymmetrical muscle-groups ; 

 but one of the most facile and persistent of all bilateral reflexes resulting from 

 unilateral stimulation is the adduction of both thighs, a bilaterally symmetrical 

 movement. 



In the trunk the spread across the median plane is not so free in some animals 

 as might have been expected : the yoking is of bilaterally symmetrical muscles. 



