456 TEXT-BOOK OF PHYSIOLOGY. 



as a rule, incomplete and of short duration, 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 lo'ng period of time after re- 

 moval of the brain, and therefore is well adapted for the study of 

 reflex actions. 



The division of the spinal cord can be readily effected by inserting 

 a spear-shaped knife between the occipital bone and the atlas. The 

 skin, occipito-atlantal membrane, and medulla can be divided with 

 one plunge of the knife. The brain can then be destroyed by the 

 insertion of a fine wire into the brain cavity. 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 it 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 

 exhibit spontaneous movements. Any stimulus applied to the skin, 

 however, provided it is of sufficient intensity, will be followed by a 

 more or less pronounced movement. Mechanic, chemic and 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 movement 

 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 purposive and defensive in character, 

 can be produced. The coordinated and purposive character of the 

 movements exhibited by a brainless frog led Pfliiger to the assump- 

 tion that the spinal cord in this as well as in other cold-blooded ani- 

 mals is possessed of sensorial functions, is 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 



