396 THE NERVOUS SYSTEM. 



The occurrence of movements under the influence of the 

 spinal cord, and independent of the will, is well exemplified 

 in the acts of swallowing, in which a portion of food carried 

 by voluntary efforts into the fauces, is conveyed by successive 

 involuntary contractions of the constrictors of the pharynx 

 and muscular walls of the oesophagus into the stomach. These 

 contractions are excited by the stimulus of the food on the 

 centripetal nerves of the pharynx and oesophagus Being first 

 conducted to the spinal cord and medulla oblongata, and 

 thence reflected through the motor nerves of these parts. All 

 these movements of the pharynx and oesophagus are involun- 

 tary ; the will cannot arrest them or modify them ; and though 

 the mind has a certain consciousness of the food passing, which 

 becomes less as the food passes further, yet that this is not 

 necessary to the act of deglutition, is shown by its occurring 

 when the influence of the mind is completely removed ; as 

 when food is introduced into the fauces or pharynx during a 

 state of complete coma, or in a brainless animal. 



So also, for example, under the influence of the spinal cord, 

 the involuntary and unfelt muscular contraction of the sphinc- 

 ter ani is maintained when the mind is completely inactive, as 

 in deep sleep, but ceases when the lower part of the cord is 

 destroyed, and cannot be maintained by the will. 



The independence of the mind manifested by the reflecting 

 power of the cord, is further shown in the perfect occurrence 

 of the reflex movements when the spinal cord and the brain 

 are disconnected, as in decapitated animals, and in cases of 

 injuries or diseases so affecting the spinal cord as to divide or 

 disorganize its whole thickness at any part whose perfection 

 is not essential to life. Thus, when the head of a lizard is cut 

 off, the trunk remains standing on the feet, and the body 

 writhes when the skin is irritated. If the animal be cut in 

 two, the lower portion can be excited to motion as well as the 

 upper portion ; the tail may be divided into several segments, 

 and each segment, in which any portion of spinal cord is con- 

 tained, contracts on the slightest touch ; even the extremity 

 of the tail moves as before, as soon as it is touched. All the 

 portions of the animal in which these movements can be ex- 

 cited, contain some part of the spinal cord ; and it is evidently 

 the cause of the motions excited by touching the surface ; for 

 they cannot be excited in parts of the animal, however large, 

 if no part of the cord is contained in them. Mechanical irri- 

 tation of the skin excites not the slightest motion in the leg 

 when it is separated from the body ; yet the extremity of the 

 tail moves as soon as it is touched. The same power of the 

 spinal cord in reflecting impressions will cause an eel, or a 



