THE SPINAL CORD. 509 



by stimulation of the sole of the foot; it involves the integrity of the 

 reflex arc through the second and third sacral nerves. 



2. Gluteal reflex, consisting of contraction of the glutei muscles when the skin 



over the buttock is stimulated; it takes place through the segments 

 giving origin to the fourth and fifth lumbar nerves. 



3. Cremasteric reflex, consisting of a contraction of the cremaster muscle 



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

 on the inner side of the thigh is stimulated; it depends upon the integ- 

 rity of the segments giving 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; its 

 production requires the integrity of the spinal segments from the 

 eight to the twelfth thoracic nerves. 



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 requires the integrity of the cord between 

 the fourth and seventh thoracic nerves. 



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



the skin between the scapulae is stimulated; it depends upon the in- 

 tegrity of the cord between the fifth cervical and third thoracic nerves. 

 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,' 1 '' are characterized by the contraction of a 

 muscle and are elicited by a sharp tap on its tendon. They are also of much 

 value in the diagnosis of lesions of the cord. 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 of the afferent nerves, the muscle spindles. 

 When the muscle is passively extended, as it is when the reflex is to be 

 elicited, there is an exaltation of the tonus and an increase in the irri- 

 tability. To this condition of the muscle due to passive tension, the term 

 myotatic irritability has been given. If the muscle extension be now sud- 

 denly 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 following are the principal forms of the tendon reflexes : 

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

 by a contraction of the extensor muscles of the thigh imparting to the leg a 

 forward movement when the ligamentum patellae is struck between the 

 patella and tibia. This reflex is best observed 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 degeneration of the cord; it is absent in 

 locomotor ataxia and in atrophic lesions of the anterior gray cornua. 



