THE SPINAL CORD AS A REFLEX CENTRE :ni 



find that the method of transection, whether by tearing across the cord or 

 cutting it with a minimum disturbance, alters appreciably the amount of 

 shock displayed by the segment of the cord situated below the lesion. On 

 the other hand, if in a dog, which has undergone transection of the cord in the 

 lower cervical region and has been allowed sufficient time to recover from the 

 shock, a second transection be carried out two or three segments below the 

 site of the first operation, the influence of the second section is hardly notice- 

 able on the lower segment of the cord. Apparently then the chief factor in 

 determining shock in all those centres situated aborally of the lesion is the 

 cutting off of the impulses which are continually streaming down from the 

 higher centres and from the great sense-organs connected with the anterior 

 portions of the nervous system. With every rise in the animal scale the im- 

 pressions received by the special senses take an increasing part in the deter- 

 mining of all the reactions of the body, so that we might expect the effect of 

 cutting off the impulses from the higher centres to be greater, the higher in the 

 scale of animal life is the animal on which the experiment is carried out. 



The state of profound shock produced in the spinal cord by the operation 

 passes off gradually. The blood pressure, which may have fallen to 40 or 

 50 mm. Hg., rises within two or three days to its normal height, i.e. 90 to 

 110 mm. Hg. The sphincter muscles of the anus gradually recover their 

 tone, and within a short time the reflex evacuation of the bladder and rectum 

 may occur as in a normal animal. The skeletal muscles recover their tone 

 within a few days, and after a short time co-ordinated movements can be 

 brought about in the trunk and limbs by appropriate stimulation of sensory 

 surfaces. At first the reactions thus produced are feeble and the reflex is 

 rapidly fatigued. Of these reflexes those excited by nocuous or painful stimuli 

 are the first to make their appearance ; a little later are seen those due to 

 stimuli affecting the tactile organs in the skin, or the sense-organs of deep 

 sensibility situated round the bones and joints and excited by deep pressure 

 or changes in posture of the limbs. 



In a dog which has undergone complete cervical transection two or three 

 months previously, the tone of the muscles is somewhat increased. Although 

 the dog is unable to walk, if it be raised and given a little push forward, so 

 as to stretch the extensor muscles of its hind limbs, it may take two or three 

 steps forward before its legs collapse. Although the locomotor apparatus 

 is present, the nexus is lacking which determines the regulation of these move- 

 ments through the organs of static sense, so that the spinal movements are 

 insufficient to maintain the animal in such a position that a line drawn verti- 

 cally from its centre of gravity shall fall between its points of support. On 

 the other hand, swimming movements may be carried out regularly. The 

 frog deprived of its brain can swim like a normal animal, but in consequence 

 of the depression of its head tends to swim ever deeper in the water. 

 ' spinal ' dog be held up by the fore limbs, the hind limbs nearly always enter 

 into alternating movements of flexion and extension (' mark time ' move- 

 ments), the two limbs acting alternately as in normal progression, 

 stimuli in this case seem to be started by the stretching of the skin and other 



