502 THE NERVOUS SYSTEM. 



spinal cord in reflecting, is different in kind from that of 

 all other nervous centres. 



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 stimu- 

 lus 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 involuntary ; 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 neces- 

 sary 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 sphincter ani is maintained when the mind is com- 

 pletely 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 reflect- 

 ing power of the cord, is further shown in the most 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 ex- 



