53 8 TEXT-BOOK OF PHYSIOLOGY 



owing to disturbances of the circulation and respiration and the consequent 

 loss of tissue irritability. In frogs and in cold-blooded animals generally, 

 the spinal cord retains its irritability for a long period of time after removal 

 of the brain, and therefore is well adapted to the study of reflex actions. 



The separation of the spinal cord from the brain is readily effected by 

 destroying the medulla oblongata. This can be done by inserting a pin 

 through the skin and the occipito-atlantal membrane covering the space be- 

 tween the occipital bone and the atlas, until it strikes the bodies of the 

 vertebrae below. If the pin is properly directed it passes through the med- 

 ulla. Care should be taken to avoid injury to the blood-vessels on either 

 side. The brain itself should then be destroyed, so as to remove all con- 

 sciousness, by inserting the pin into the brain cavity through the foramen 

 magnum, and giving it a few rotatory movements. 



A frog so prepared, and placed on the table and allowed to remain at 

 rest for a few moments until the shock of the operation passes away, will 

 draw the limbs close to the body and 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 leg, to be succeeded in a 

 short time by extension. Stronger solutions will produce more extensive 

 and vigorous 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 is, within limits, 

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

 action of the stimulus to definite but different areas of the skin a great variety 

 of movements, more or less complex and coordinated and apparently pur- 

 posive and defensive in character, can be produced. The coordinated and 

 purposive character of the movements exhibited by a brainless frog led 

 Pfliiger 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. 



In warm-blooded animals similar results may be obtained for a short 

 time after division of the cord, especially if artificial respiration is maintained 

 and the circulation of the blood continued. The cord will then retain its 

 irritability for some time. If the conditions of experimentation were favor- 



