THE SPINAL CORD 



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is acted on by two different nerve-fibers, one of which when stimulated 

 augments the activity of one and inhibits the activity of the other. Thus 

 in each reflex action one and the same stimulus will augment the activity 

 of the flexor center and inhibit the activity of the extensor center; and 

 conversely if the stimulus augments the activity of the extensor center it 

 inhibits the activity of the flexor center. Other and more complicated 

 instances of reciprocal activity of synergic and antagonistic muscle 

 groups are observed in the skeletal muscles both during station and 

 progression and which involve accordingly more complicated mechanisms 

 for their innervation. Reciprocal innervation, sometimes central, some- 

 times peripheral, is observed in the action of antagonistic groups of 

 muscles, such as the detrusor and sphincter muscles of the bladder, of 

 the rectum, the dilator and sphincter muscles of the iris; the innervation 

 in these and other instances falls under the term reciprocal innervation. 

 Special Reflex Actions. Among the reflexes connected with the 

 more superficial portions of the body there are some which are so frequently 

 either increased or diminished in pathologic conditions of the spinal cord 

 that their study affords valuable indications as to the seat and character of 

 the lesions. They may be divided into: 



1. The skin or superficial reflexes. 



2. The tendon or deep reflexes. 



3. The organ reflexes. 



The skin or superficial reflexes, characterized by contraction of under- 

 lying muscles, are induced by stimulation of the afferent nerve-endings of 

 the skin e.g., by pricking, pinching, scratching, etc. The following are the 

 principal skin reflexes: 



1. Plantar reflex, consisting of contraction of the muscles of the foot, induced 



by stimulation of the sole of the foot; it takes place through the seg- 

 ments of the cord which give rise to 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 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 hi 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 



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