914 THE MESENCEPHALON. 



movements proceed regularly, and the pulse remains quiet and 

 full, but certain groups of muscles become rigid. The elbows and 

 knees are stiffly extended, and do not allow of the usually easily 

 made passive flexion. The tail is stiff and straight instead of pliable 

 and drooping ; the neck and head retracted. The condition seems more 

 accentuated in the fore-limb than in the hind. Wrist and ankle are 

 much less affected, the digits not at all. Passive movement of the limb 

 hastens the development of this prolonged spasm and re-develops it when 

 it has subsided. The spasm seems dependent on an action perhaps a 

 tonic action of the afferent spinal roots of the region implicated by 

 the spasm. Section of the afferent roots of the arm, prior or subsequent 

 to the transection of the mesencephalon, prevents or abolishes the 

 rigidity in it. The influence thus based upon the afferent roots seems 

 to descend from a region above the lowest third of the bulb, for uni- 

 lateral transection at that level abolishes the rigidity or prevents it, 

 and on the same side as the section. The inhibition influence from the 

 optic lobes of the frog has been noted to be crossed. 1 



In this condition of decerebrate rigidity reflexes can be elicited 

 which actuate the neck, trunk, and the four limbs as a whole, and treat 

 them as components of a single motile apparatus. There must, therefore, 

 in the mammal exist behind the diencephalon a nervous mechanism, 

 which can co-ordinate the action of the great regions of skeletal muscu- 

 lature (including the tail), and the character of the combined move- 

 ments resulting leaves little room for doubt that the combination is 

 subservient of quadrupedal progression. The combined movement 

 treats the pair of fore-limbs in such a manner that the one is flexed 

 and protracted, the other is extended and retracted at the same 

 time, and similarly when the one limb is flexed at hip and knee, its 

 fellow is extended at those joints. Further, there is a diagonal relation 

 between the limbs in the combined movement of such a kind that a 

 fore-limb and the diagonal hind-limb are flexed together or extended 

 together. It results, therefore, that there are two possible postures that 

 the reflex can produce as regards the limbs (1) Left fore-limb and right 

 hind-limb flexed, with right fore-limb and left hind-limb extended ; (2) 

 left fore-limb, left hind-limb extended, with right fore-limb and left hind- 

 limb flexed. Which of these is produced is determined by the seat of 

 stimulus. The stimulus regularly provokes flexion in the limb stimu- 

 lated as a starting-point, and affects the other three limbs in the way 

 above stated. These reflexes are frequently " alternating " in character, 

 so that successive flexions and extensions of the limbs follow in 

 sequence, extension and retraction succeeding to flexion and protraction ; 

 and in this alternation the diagonal symmetry characteristic of the 

 primary movement is maintained through the succession. The rigidity 

 ensues after a semisection of the region, and is most marked on- 

 the side of section. It can then be inhibited in various parts of the 

 musculature, by stimulation of appropriate parts of the Eolandic area 

 of the crossed hemisphere. 



A point of importance attaching to decerebrate rigidity is the field 

 it offers for examination of the play of inhibition, and this is exemplified 

 in the execution of the combined limb reflexes just mentioned. Thus, 

 to give a single example, if the skin of the left hand be the locus of 

 excitation, there ensues inhibition of the left triceps brachii, with con- 



1 Langendorff, Arch. f. Physiol., Leipzig, 1877. 



