v SPINAL COED AND NEEVES 315 



the opposite side from that excited, are examples of reflexes 

 opposed to this law. 



(d) The irradiation of reflexes spreads more easily towards than 

 away from the medulla oblongata, i.e. downwards from the cranial 

 nerves, upwards from the spinal nerves. When the excitation of 

 a sensory cranial nerve spreads reflexly to a motor nerve, this 

 nerve, according to Pfliiger, is at approximately the same level in 

 the central organ as, or lower but never higher than, the sensory 

 nerve. If the excitation spreads farther the direction of irradiation 

 is always downwards, towards the bulb. Thus on exciting the 

 optic nerve the pupil contracts, i.e. the impulse passes from the 

 optic to the oculomotor nerve, and thence, from above downwards, 

 towards the bulb. In the cord, on the other hand, the motor nerve 

 first excited is at the same level as the sensory root through which 

 the excitation passes, but when the reflex spreads the path of 

 irradiation is, according to Pfliiger, always upwards, towards the 

 bulb. Thus excitation of the finger evokes reflexes in the cervical 

 region of the cord, and on spreading, the excitation passes through 

 the cervical cord to the nuclei of the spinal accessory, vagus, etc., 

 and not to the thoracic and lumbar parts of the cord. It is only 

 after reaching the bulb that the excitation is able to spread down- 

 wards to the lumbo-sacral region. 



This law is the most disputed of all, as it presents the most 

 exceptions. It contradicts the observations of Sherrington, who 

 observed in mammals that in the majority of instances irradiation 

 spreads more easily down than up the cord. It is easier to obtain 

 reflex movements of the limbs and tail by excitation of the skin of 

 the pinna than the reverse ; it is more difficult to elicit a move- 

 ment of the fore-limb by excitation of the hind-limb than the 

 reverse. 



Sherrington endeavoured to determine the salient features of 

 long intraspinal reflexes in normal mammals and in those in which 

 the cord is severed from the brain. The animal is supported 

 freely from above with its spinal axis horizontal, so that the 

 attitude of the limbs is determined by gravitation. On exciting 

 different areas of skin under these conditions, he found that 

 certain areas discharge reflexes to the skeletal musculature more 

 easily than others. These areas are the soles, the palms, the 

 pinnae, the tail, the perineal region ; and with the exception of 

 the last these areas are those which possess the greatest range of 

 motility. Irradiation from these reflexigenous areas takes place in 

 a definite order. If, e.g., in the cat with isolated cord (Sherrington's 

 " spinal cat ") the left hind-limb is stimulated, movement is excited 

 in that leg, which spreads to the tail, then to the right hind-limb, 

 lastly to the left fore-limb. If the left fore-limb is stimulated, the 

 movement spreads thence to the left hind-limb, the tail, the right 

 hind-limb, and lastly the right fore-limb. If the left pinna be 



