ACTION AS A NERVOUS CENTRE. 465 



The spinal cord by itself is not sufficient to produce the muscular 

 actions required for standing and locomotion; since we know that any 

 sudden lesion which deeply injures the brain, or cuts off the medulla 

 oblongata, or divides the spinal cord above the cervical or lumbar enlarge- 

 ments, either in mammalia or in man, instantly destroys the power of 

 standing upright, or of making any effective movements of locomotion. 

 In the frog, an attitude very similar to the natural one is often preserved 

 after decapitation, since the body rests by most of its under surface 

 upon the ground ; and the contact of the integument, through the reflex 

 action of the spinal cord, brings the limbs underneath it in a flexed 

 position. If such a frog be held suspended in the air, the limbs hang 

 down in a relaxed condition, and again assume the natural attitude of 

 flexion, when replaced in contact with a hard surface ; and, according to 

 Poincare, 1 it can sometimes be made to execute a series of leaps, each 

 concussion, as the body strikes the ground, giving a fresh stimulus for 

 another reflex movement of extension in the limbs. But in the case of 

 the frog and of the amphibious reptiles generally, the muscular actions 

 required, both for the attitude and for locomotion, are of the simplest 

 character. In the warm-blooded quadrupeds and in man, on the other 

 hand, the act of volition is essential for either standing or progression; 

 and both these powers are abolished by cutting off the communication 

 of the spinal cord with the brain. 



But, although the voluntary impulse is necessary to produce the acts 

 of standing or walking, it does not seem to be concerned in the details 

 of their mechanism. Once excited, the nervous action by which walking 

 is accomplished may be kept up without any mental effort, the attention 

 being directed to something else. All we have to do is, to commence 

 the process by an act of volition, and the requisite nervous machinery 

 is at once set in motion. If we decide to turn a corner, all the muscular 

 combinations necessary for that purpose are effected without the imme- 

 diate intervention of the consciousness or the will. This secondary 

 action, by which the different motor impulses are combined in the limbs 

 and trunk, is undoubtedly dependent upon the integrity of the spinal 

 cord. 



The precise mode in which this action is accomplished is not positively 

 ascertained. The most probable explanation at present known is that 

 it is due to a constant reflex activity of the cord, by which the muscles 

 in different parts of the body and limbs are kept in the proper degree 

 of tension or relaxation ; and that the different parts of the cord are 

 united with each other for this purpose by longitudinal commissural 

 fibres which enter and leave its gray substance at successive points. 

 Some authors (Todd, Vulpian, Poincare) adopt the opinion that the 

 posterior columns constitute such longitudinal commissures. The 

 reasons for this opinion are not fully satisfactory, since anatomical 

 investigation has thus far failed to show what is the actual origin and 



1 Lecjous sur la Physiologic dn Systfeme Nervcux. Paris, 1873, p. 72. 



