v SPINAL COED AND NEEVES 337 



fowls can fly directly after decapitation. Tarchanoff s observations 

 on ducks are more interesting. After transection of the cord 

 between the 4th and 5th cervical vertebrae they can perform a 

 long series of perfectly regular swimming movements in the water, 

 both with the feet and the wings. But if placed on a table they are 

 incapable of standing upright, although they execute regular 

 alternating movements of walking with their legs. 



In man, too, complex co-ordinated reflexes of defence have 

 been observed in cases of contusion or dislocation of the cord in 

 the cervical or thoracic region. Marshall Hall describes a man 

 whose cord was crushed at the neck by a fall. There was complete 

 motor and sensory paralysis of the lower half of the body, but 

 when stimulated either with painful mechanical stimuli, or with 

 hot water, or by tickling the soles of the feet, the lower limbs 

 moved with great vigour as if the patient's cord felt the pain or 

 was aware of the tickling. 



The fully co-ordinated defensive movements carried out in 

 sleep (e.g. in response to bites of fleas or mosquitoes), and many 

 quite unconscious movements made during the waking state, 

 while the attention is otherwise occupied, are similar, and should 

 probably be classed among the purely spinal co-ordinated reflexes. 



All these instances illustrate the great complexity of the 

 spinal reflexes a complexity that cannot be explained by the 

 simple spread of excitation from the afferent nerves into adjacent 

 nerve -cells. 



An adequate theory of reflexes must throw light on the process 

 by which the centripetal or afferent excitation becomes centrifugal 

 or efferent ; it must tell us why the reflex is sometimes confined 

 to a few muscles, and at other times spreads to more muscles in 

 various combinations ; why the efferent impulses travel along 

 certain paths and not others; lastly, how the co-ordination and 

 adaptation of the reflexes to the nature and localisation of the 

 stimulus is attained. At present we can only give vague and 

 inadequate replies to these questions, though a few hypothetical 

 but certainly ingenious attempts have been made towards a partial 

 solution of the problem on the basis of the neurone theory. 



The greater or less irradiation of reflexes and the laws by 

 which they are governed, are generally explained by the more or 

 less direct and easy communication between the sensory and 

 motor neurones concerned; or the greater or less conductivity 

 along the paths formed by the fibrillary networks in the grey 

 matter. 



It is harder to explain the adaptation of the reflex to the 

 stimulus. In this connection the fact is usually cited that habit 

 facilitates the transmission and association of actions that were 

 difficult in the first place, which is possibly due to improved con- 

 ductivity along the paths. 



VOL. Ill Z 



