542 TEXT-BOOK OF PHYSIOLOGY 



nerve and, on the other hand, through collateral branches with the 

 dendrites of the efferent neurons situated at different levels. (Fig. 226.) A 

 histologic and physiologic mechanism of this character readily explains 

 how a localized stimulation can give rise to reflex actions extremely com- 

 plex in character. 



Methods of Studying Reflex Action. The reflex contractions of 

 skeletal muscles are best studied in the frog after division of the central 

 nerve system at the upper limit of the spinal cord. After this procedure 

 the spinal centers can act independently of, and uninfluenced by either 

 sensation or volitional efforts on the part of the animal. In frogs and in 

 cold-blooded animals generally, the spinal cord retains its irritability for 

 a long period of time after destruction of the brain, and therefore is well 

 adapted for the study of reflex actions. 



After the separation of the brain from the spinal cord the frog passes 

 into a condition of shock during the existence of which the muscles are 

 relaxed and flaccid. In a short time the shock to some extent passes 

 away and the spinal cord resumes its former activity. The limbs will 

 then be drawn close to the body and the frog will assume a position not 

 unlike that of a normal frog. If then the posterior limbs be extended, 

 they will immediately be drawn close to the side of the trunk in the usual 

 flexed position. If the toes are pinched with forceps, the foot will execute 

 a series of movements as if the frog were trying to free itself from the source 

 of irritation. 



If the frog be suspended, the limbs, through the force of gravity, will be 

 gradually extended and hang down freely. In this, as in the sitting position, 

 the animal will remain perfectly quiet and will not execute spontaneous 

 movements. Any stimulus applied to the skin, however, provided it is of 

 sufficient intensity, will be followed by a more or less pronounced move- 

 ment. Mechanic, chemic, or electric stimuli applied to any part of the 

 skin will call forth the characteristic reflex movements. Chemic stimuli 

 such as weak solutions of sulphuric or acetic acid placed on the toes will be 

 followed by feeble flexion of the corresponding foot, leg or even thigh, 

 to be succeeded in a short time by extension. Stronger solutions will 

 produce more extensive and pronounced flexion and extension movements, 

 the foot at the same time being rubbed against the thigh, apparently 

 for the purpose of freeing it from the irritant. Similar phenomena follow 

 the application of the acid to the fingers or the trunk. As a rule, the 

 extent and complexity of the movements are within limits, proportional 

 to the strength of the stimulus. By limiting the sphere of action of the 

 stimulus to definite areas of the skin a great variety of movements, 

 more or less complex and coordinated and apparently purposive and 

 defensive in character, can be produced. The coordinated and pur- 

 posive character of the movements exhibited by a brainless frog led 

 Pfluger to the assumption that the spinal cord in this as well as in other cold- 

 blooded animals is possessed of sensorial functions, and endowed with 

 rudimentary consciousness. This view, however, is not generally accepted, 

 the movement being attributed to specialized mechanisms in the cord, 

 partially inherited, which permit of one and the same movement with 

 mechanic regularity and precision, so long as the conditions of the experi- 

 ment remain the same. 



