THE CENTRAL NERVOUS SYSTEM. 65 



The normal maintenance of this tone is really a reflex action and 

 is dependent on afferent impulses, which in the mammal originate 

 chiefly in the muscles themselves. It is abolished, therefore, not 

 only by section of motor nerves, but also by section of the posterior 

 nerve roots containing the afferent fibres coming from the muscles. 

 Impulses passing down the spinal cord from the brain may also control 

 and modify muscular tone ; and this is sometimes greatly increased, 

 when the passage of these impulses is prevented by injury to the spinal 

 cord in animals, or by disease of the pyramidal tract in man. 



The afferent impulses passing from muscles to the spinal cord may 

 not only bring about reflex actions, but play an important part in the 

 co-ordination of reflex actions brought about by external stimuli. If the 

 afferent nerves from the muscles of a limb are divided, the movements 

 of that limb are inco-ordinate (ataxic), even though cutaneous sensation 

 is not interfered with. On the contrary, division of the cutaneous 

 nerves has but little effect upon the co-ordination of muscular move- 

 ments, provided the afferent muscular fibres are intact. For instance, a 

 cat, even after the division of all the cutaneous nerves to its four paws, 

 is still able to balance itself almost as accurately as a normal animal. 



The Visceral Spinal Reflexes. Local centres exist in the spinal cord 

 associated with reflexes connected with the blood-vessels and sweat glands. 



Centres also exist in the lumbo-sacral region of the spinal cord for 

 the functions of micturition, defsecation, erection, and parturition. All 

 these centres are normally controlled to a greater or less degree by the 

 higher centres, but the reflex function of each can be carried out when 

 all connection with the higher centres has been severed. 



SPINAL SHOCK. 



This is a condition, following transverse section of the spinal cord, 

 in which the reflex functions of the cord are abolished in the part 

 posterior to the lesion. It is seen in its simplest form in the 

 frog. If the spinal cord of a frog is divided just posterior to the 

 medulla, the muscles of the limbs become flaccid and remain in this 

 condition for about half an hour. During that time no reflex can be 

 elicited in the limbs. As the shock passes off, muscular tone returns, 

 the animal assumes a fairly normal position, and reflex muscular con- 

 tractions can once more be evoked J In mammals the condition of 

 spinal shock is more prolonged. Section of the spinal cord below the 

 origin of the phrenic nerves results in loss of muscle tone and of 

 vascular tone in the body posterior to the lesion. The sphincters of 

 the anus and bladder are relaxed, the reflexes for defsecation and 

 micturition are abolished, and muscular reflexes cannot be elicited. 



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