406 THE NERVOUS SYSTEM. 



the case of reflex action observed by Robin in a decapitated criminal, 

 the effect of irritating the skin over the chest was a flexion and inward 

 rotation of the arm and forearm; and this necessarily brought the 

 hand near the point irritated. It is evident that the connection of 

 sensitive fibres with motor fibres, through the gray substance of the 

 cord, may be such as to call into action particular muscles, without 

 the intervention of consciousness or voluntary impulse. This is the 

 character of the reflex action of the spinal cord. 



As a general rule, movements of flexion are adapted to protect the 

 part from external injury, and are excited by moderate causes ; those 

 of extension are calculated to repel the foreign substance or to escape 

 from it by moving the whole body, and are called out by unusual or 

 excessive stimulus. The defensive character of these movements is 

 frequently manifest, in a state of health, when the brain takes no part 

 in their production. If the surface of the skin be unexpectedly brought 

 in contact with a heated body, the injured part is often withdrawn by 

 a rapid and convulsive movement, before we feel the pain, or fairly 

 understand the cause of the involuntary act. Whenever the body 

 accidentally loses its balance, the limbs are thrown into a flexed posi- 

 tion, calculated to protect- the exposed parts and to break the fall, by 

 a similar involuntary movement. Notwithstanding, therefore, the evi- 

 dent utility of these actions, they have no intentional character, and 

 are performed without distinct consciousness of their object. 



The spinal cord has also an important action in regard to attitude 

 and locomotion. The preservation of the attitude alone requires the 

 harmonious action of many different muscles, all of which contribute to 

 the position of the body. This is especially the case in man, where, in 

 the standing posture, the body is balanced upon its narrow supports, 

 preserving its equilibrium without attention or fatigue. In locomotion, 

 the flexors and extensors of the limbs are associated in a manner pecu- 

 liar to each species of animal ; and in man the balancing of the body, 

 in progression, requires a still more extensive muscular combination 

 than when at rest. 



The spinal cord is not sufficient by itself for the acts of standing 

 and locomotion ; since a sudden lesion which deeply injures the brain 

 or medulla oblongata, or the spinal cord above the cervical or lumbar 

 enlargements, at once destroys the power of standing upright, or of 

 making any effective movements of locomotion. In the frog, a very 

 natural attitude is often preserved after decapitation, since the body 

 rests by most of its under surface upon the ground ; and this, through 

 the reflex action of the spinal cord, brings the limbs underneath it in 

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

 hang down relaxed, but resume the attitude of flexion when placed in 

 contact with a hard surface ; and, according to Poincare,* the frog can 

 sometimes be made to execute a series of leaps, each concussion, as 



* Le9ons sur la Physiologie du Systbme Nerveux. Paris, 1873, p. 72. 



