THE SPINAL REFLEXES. 729 



the vasomotor centers in the medulla and spinal cord. Mammals may die from 

 asphyxia during the attack; although after large doses death results from spinal 



Earalysis when the convulsions subside early. Fowl are as a rule immune to 

 lirly large doses. 



Elicitation of the Reflexes. Feeble stimuli that, applied once, are in- 

 capable of exciting a reflex may do so after repeated application. Under 

 such circumstances a summation of the individual stimuli takes place 

 in the spinal cord. 



In order to obtain such a result three feeble stimuli in a second suffice; the 

 best results are obtained from sixteen in a second, and beyond this no increase 

 in the intensity of the effects is possible. Nevertheless, stimuli (induction-shocks) 

 within other limits, namely an interval of from 0.05 to 0.04 second, have been 

 found effective. W. Stirling has shown that the reflexes are probably due to 

 repetition of the impulses sent to the nervous centers. 



Diffusion of Reflexes. Pfliiger has established the law according to which 

 the diffusion of reflexes takes place: (i) The reflex movement takes place first on 

 the same side as that on which the sensory nerve is stimulated, and only those 

 muscles are thrown into action whose nerves arise from the same level of the cord. 

 (2) If the reflex extends to the other side, it always occurs, as an asso- 

 ciated movement only in the muscles that are already contracted on the primary 

 side. (3) If the intensity of the spasm is different on the two sides, the move- 

 ments are stronger on the primary side. (4) On diffusion of the reflex irritation 

 to adjacent motor nerves those are involved always that are situated in the direc- 

 tion toward the medulla oblongata. Sherrington, however, has observed also 

 diffusion of reflexes in a caudal direction. (5) Finally, all of the muscles become 

 involved in the spasm. 



In exceptional cases, however, deviations from these rules occur. If, for 

 example, the region of the eye in a frog, after extirpation of the cerebrum, be 

 stroked, a reflex in the hind leg of the opposite side often occurs. Tickling the 

 foreleg of decerebrated tritons, lizards, turtles, and deeply narcotized dogs and 

 cats, often causes a movement of the hind leg on the opposite side. These mani- 

 festations have been called crossed reflexes. If in animals a section be made 

 throughout the length of the spinal cord in the median line the reflexes will natur- 

 ally remain unilateral. 



Every sensory root has, in its individual spinal segment, a motor reflex path, 

 which offers the least resistance to the discharge (simple reflex). There are, how- 

 ever, also reflex paths into adjacent and remote segments; there is a functional 

 relation between certain motor-cell groups and certain muscle-groups that act 

 synergistically. The long association-paths in the spinal cord are primarily un- 

 decussated; crossed conduction appears, however, to exist between segments not 

 widely separated. The crossed reflex path can be traversed with varying ease 

 at different levels in the cord. The reflex readily passes in a crossed direction 

 from the anterior to the posterior extremity; on the other hand, from the pos- 

 terior extremity more readily to that of the opposite side. 



The reflex can be conveyed within various levels of the cord. On applying 

 feeble stimuli to the leg of a decerebrated frog, the reflex transference takes place 

 at the junction of the cervical cord and the medulla; on applying stronger stimuli 

 transference takes place at the lower portion of the spinal cord, which can be 

 stimulated reflexly with greater difficulty. If alternating hemisections of the cord 

 be made, the reflex irritation may nevertheless be propagated upward, passing 

 through both sides of the cord in a serpentine manner. The greater the number 

 of sections, the stronger must be the irritation of the sensory nerves. 



3 . The widespread coordinated reflex is characterized by the occurrence 

 in entire and even different groups of muscles of complex movements 

 having a purposive character or resembling voluntary movements and 

 following irritation of a sensory nerve. 



The observations are made either on cold-blooded animals (such as decapitated 

 frogs, lizards, or eels) or on mammals, the four arteries passing to the brain being 

 ligated (artificial respiration being maintained), so that the brain is rendered incap- 

 able of functionating. Reflexes involving the lower portion of the spinal cord may 

 be studied also in animals (or man) after transverse section of the spinal cord in 



