THE SPINAL CORD AS A REFLEX CENTER 



595 



destroyed by this procedure which are normally mediated by the 

 spinal cord. Thus, the large number of sympathetic responses 

 continue even in the absence of the cord and the same holds true of 

 those accomplished with the help of the cranial nerves, provided, of 

 course, that the region of the medulla oblongata has been left intact. 



In the frog, the spinal cord extends backward as far as the ninth 

 vertebra, namely, to the prominence upon the dorsal aspect of its 

 body. The tenth vertebra, or urostyle, continues onward from here 

 and forms the dorsal wall of the long extended ab- 

 domen and pelvis. By cutting transversely across 

 the cord, beginning at the level of the first vetebra, 

 it is possible to show that the reflexes from the 

 hind limbs are not abolished until the level of the 

 cartilage between the sixth and seventh vertebrae 

 has been reached. Any section distally to this 

 point of the cord destroys the aforesaid reflexes 

 immediately. The conclusion must, therefore, be 

 made that the reflex center for the hind limbs is 

 situated opposite to the seventh and eighth verte- 

 brae (Fig. 294). It is generally designated as the 

 ''sciatic center," because the paths which connect 

 it with the periphery are collected on each side in 

 one bundle, known as the sciatic nerve. The latter 

 arises by three roots and it can be shown by stimu- 

 lation with weak electrical currents that these 

 radicles possess a somewhat different function, be- 

 cause they innervate different groups of muscles 

 and thus give rise to several specific movements 

 of the leg. With the aid of very delicate electrodes, 

 it can also be proved that a similar localization of 

 function is present in the sciatic center itself. 



This method of dividing the spinal cord at 

 different levels has also proved that the centers for 

 the muscles of the abdomen are situated anteriorly 

 to the sciatic center and that the center for the fore 

 limbs is located anteriorly to these. Several reflex 

 and automatic centers are also found in the me- 

 dulla oblongata, namely, those controlling the car- 

 diac, respiratory and vasomotor activities. It is evident, therefore, 

 that the spinal cord of the frog and allied animals contains a series of 

 centers for simple reflex action and that a segmentalism exists in 

 these animals which closely approaches that found in the vermes and 

 crustacese. 



Spinal Reflexes in the Mammals. — If the attempt is made to 

 pursue similar methods of localization in the mammals, we are 

 immediately confronted by several difficulties, one of which is 

 the much greater susceptibility and sensitiveness of the nervous 



Fig. 294.— Dia- 

 gram TO SHOW THE 



Position of the Re- 

 flex Centers in the 

 Spinal Cord of the 

 Frog. 



BC and BN, 

 Brachial center and 

 nerve; A, center for 

 the parts of the 

 trunk; SC and SN , 

 sciatic center and 

 nerve. The num- 

 bers indicate the 

 different vertebrae. 



