146 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



less than normal. Its power of preserving a constant body tem- 

 perature 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, less- 

 ened, 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. According 

 to Sherrington, the parts of this muscular mass chiefly concerned 

 are the m. vastus medialis and m. femoralis. In order to obtain 

 the muscular response it is usually necessary to put the quadriceps 

 under some tension by flexion of the leg. This end is obtained 

 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 of 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,t 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 . Psychiatric," 1875, vol. v. 



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



