REFLEX ACTIONS. 155 



constant body temperature was more limited than in the normal 

 animal, and the susceptibility to inflammatory disturbances in the 

 visceral organs was greatly increased. It seems evident, from these 

 facts, that, although the animal was living, its power of adaptation to 

 marked changes in the external or internal environment was greatly 

 lessened, and this fact illustrates well the great general importance 

 of the spinal cord and brain as reflex centers controlling the nutri- 

 tion and co-ordinated activities of the body tissues and organs. 

 This control is necessary under normal conditions for the success- 

 ful combination of the activities of the various organs.- A large 

 part of this control is doubtless dependent upon the regulation of 

 the blood supply to the various organs. The mechanism by which 

 this is effected and the parts played by the cord and the brain 

 (medulla oblongata), respectively, will be described in the section 

 on Circulation. 



Knee-jerk. Knee-jerk or knee-kick is the name commonly 

 given to the jerk of the foot when a light blow is struck upon the 

 patellar ligament just below the knee. The jerk of the foot is 

 due to a contraction of the quadriceps femoris muscle. Accord- 

 ing to Sherrington, the parts of this muscular mass chiefly 

 concerned are the m. vastus medialis and m. vastus intermedius. 

 In order to obtain the muscular response it is usually neces- 

 sary to put the quadriceps under some tension by flexion of the 

 leg. This end is achieved most readily by crossing the knees 

 or by allowing the leg to hang freely when sitting on the edge 

 of a bench or table. Under such circumstances the jerk is 

 obtained in the great majority of normal persons, and this 

 fact has made it an important diagnostic sign in many diseases 

 of the spinal cord. The importance of the reaction for such 

 purposes was first brought out by the work of Erb and Westphal * 

 in 1875. 



Reinforcement of the Knee-jerk. It was first shown by 

 Jendrassik (1883) that the extent of the jerk may be greatly aug- 

 mented if, at the time the blow is struck upon the tendon, a strong 

 voluntary movement is made by the individual, such as squeezing 

 the hands together tightly or clenching the jaws. This phenomenon 

 was studied carefully in this country by Mitchell and Lewis, f who 

 ascertained that a similar augmentation may be produced by giving 

 the individual a simultaneous sensory stimulation. They desig- 

 nated the phenomenon as a reinforcement, and this name is gen- 

 erally employed by English writers, although occasionally the term 

 "Bahnung," introduced by Exner to describe a similar phenom- 

 enon, is also used. It is found that by a reinforcement the knee- 



* Erb and Westphal, "Archiv f. Psychiatrie," 1875, vol. v. 



t Mitchell and Lewis, "American Journal of Med. Sciences," 92, 363, 1886. 



