326 Comparative Animal Physiology 



tion, resonance, and plasticity merely cover our ignorance of the nature of 

 nervous integration. 



In mammals the spinal cord shows less autonomy, and shock following 

 cord section lasts much longer in higher than in lower orders. ^^^ When the 

 cord of a cat or dog is transected in the upper thoracic region so that respira- 

 tion continues, all motor reflexes are at first depressed. Flexor reflexes return 

 in a cat in a few hours, extensor reflexes after several days. In man spinal 

 shock, when even flexor responses are unobtainable, lasts 1 to 6 weeks after 

 spinal transection. It was formerly noted that good stretch reflexes did not 

 return, and that for many weeks a light tactile stimulus might elicit flexor 

 spasms of the legs along with defecation, urination, and sweating, the so- 

 called "mass reflex." Observations on a large number of war casualties who 

 received superior care showed that flexor spasms predominate from six weeks 

 to one year, and that extensor spasms predominate thereafter, with much 

 individual variability. Many reflexes reappear, and the spasms can be stop- 

 ped by cutting dorsal roots.-^^ Spinal shock results from interruption of the 

 corticospinal system, specifically the pyramid tract, and is greater in chim- 

 panzees than in baboons and macaques, and less in monkeys.^^^ 



Reflex potentialities of the cord are well seen in animals such as cats and 

 dogs, in which recovery from shock occurs after a few days or weeks. A 

 spinal cat or dog cannot stand, yet when suspended it may make rhythmic 

 treading motions. A variety of intersegmental movements, both contralateral 

 and ipsilateral, can be elicited, many of obviously primitive protective value 

 to the organism. Single limb reflexes involving many muscles, such as the 

 rhythmic scratch response to a stimulus to the skin of the back, show much 

 central coordination. Visceral functions such as urination and defecation are 

 elicited reflexly; when one leg is warmed vasodilatation may occur in the op- 

 posite one. Sexual functions such as erection of the penis and ejaculation, 

 and assumption of copulatory posture occur, and spinal bitches have been 

 impregnated and delivered of puppies after a normal gestation period.^ ^^ 



Short pieces of the cord,*^- isolated by two cuts and appropriate root sec- 

 tion, are capable of some reflexes. The sensory areas of single dorsal roots 

 have been mapped^^^ in dogs. An isolated lumbo-sacral region of a puppy 

 showed no autonomous movements, although rhythmic flexions might be 

 elicited by pressure stimulation of the cord."*^^ However, deafferented sec- 

 tions of the cord and sections in curarized and narcotized cats showed much 

 "spontaneous" discharge, particularly in the ventral columns.^^ Some stim- 

 ulus seems to be necessary to integrate this "spontaneous" activity into co- 

 ordinate movements. A single sensory volley may elicit rhythmic responses 

 in the legs, even in completely deaff^erented hind limbs. Isolated segments 

 of the frog spinal cord show less electrical activity than when they are con- 

 nected to the brain, but show much more activity than when the spinal 

 nerves of the isolated segments are cut.^°^ 



The literature dealing with interaction between regions of the cord 

 and between cord and brain is very extensive. In general, this interaction in- 

 creases with increase of ascending and descending columns in higher verte- 

 brates. Lower cord centers may even inhibit higher ones, as in hyperreflexia 

 of forelimbs after lumbar spinal section, the Schiff-Sherrington phenome- 



