542 TEXT-BOOK OF PHYSIOLOGY 



and a retraction of the testicle toward the abdominal ring when the skin 

 on the inner side of the thigh is stimulated; it takes place through the 

 segments which give origin to the first and second lumbar nerves. 



4. Abdominal reflex, consisting of a contraction of the abdominal muscles 



when the skin upon the side of the abdomen is gently scratched; it takes 

 place through the spinal segments which give origin to the nerves from 

 the eighth to the twelfth thoracic. 



5. Epigastric reflex, consisting of a slight muscular contraction in the neigh- 



borhood of the epigastrium when the skin between the fourth and 

 sixth ribs is stimulated; it takes place through the segments of the cord 

 which give origin to the nerves from the fourth to the seventh thoracic 

 inclusive. 



6. Scapular reflex consisting of a contraction of the scapular muscles when 



the skin between the scapulae is stimulated; it takes place through the 

 segments of the cord which give rise to the nerves from the fifth cervical 

 to the third thoracic inclusive. 



The skin or superficial reflexes, though variable, are generally present in 

 health. They are increased or exaggerated when the gray matter of the 

 cord is abnormally excited, as in tetanus, strychnin-poisoning, and disease 

 of the lateral columns. 



The so-called "tendon reflexes," are characterized by a movement of cer- 

 tain parts of the body due to the contraction of certain muscles and are 

 elicited by a sharp tap on their tendons. The fundamental condition for 

 the production of the tendon reflex is a certain degree of tonus of the muscle, 

 which is a true reflex, maintained by afferent nerve impulses developed in 

 the muscle itself in consequence of its extension and hence compression of 

 the end-organs, the muscle spindles, of the afferent nerves. When the mus- 

 cle is passively extended, as it must be when the reflex is to be elicited, there 

 is an exaltation of the tonus and an increase in the irritability. To this 

 condition of the muscle due to passive tension, the term myotatic irritability 

 has been given. If the muscle extension be now suddenly increased, as it 

 is when the tendon is sharply tapped, the increased compression of the muscle 

 spindles will develop additional afferent impulses which after transmission 

 to the spinal cord will give rise to contraction of the corresponding muscle. 

 The tendon reflexes are of much value in the diagnosis of certain lesions of 

 the spinal cord. 



The following are the principal forms of the tendon reflexes : 



1. The Patellar tendon reflex or knee-jerk. This phenomenon is characterized 



by a quick extension of the leg from the knee downward, due to the 

 contraction of the extensor muscles of the thigh, when the ligamentum 

 patellae is struck between the patella and tibia. This reflex is best ob- 

 served when the legs are freely hanging over the edge of a table. The 

 patella reflex is generally present in health, being absent in only 2 per 

 cent. ; it is greatly exaggerated in lateral sclerosis, in descending degen- 

 eration of the cord; it is absent in locomotor ataxia and in atrophic 

 lesions of the anterior gray cornua. 



2. The tendo-Achillis reflex or ankle-jerk. This phenomenon is characterized 



by a flexion of the foot due to a contraction of the gastrocnemius muscle 

 when the tendo-Achillis is struck. To elicit the contraction, the leg 

 should be extended and the dorsum of the foot be pressed toward the 



