8Q 



VETERINARY PHYSIOLOGY 



synapses Avith neurons which send their axons doAvn the 

 same side of the spinal cord, and give off collaterals which 

 act upon the spinal arcs. (6) Fibres also pass to the 

 red nucleus of the cerebrum, from which fibres pass down 

 the cord to modify its action. 



B. General Action of the Arcs. 



The special action of each of these arcs may be studied 

 by observing the effect of interruption of its different 

 parts. 



1. Cerebral Arc — (1) If, in a normal frog, the attitude, 

 movements, responses to various stimuli and power of balanc- 

 ing be studied, and if (2) the anterior part of the brain, the 



A B 



Fig. 34. — A, Pigeon with the Cerebellum destroyed to show struggle to 

 maintain tlie balance ; B, Pigeon with Cerebrum removed to show 

 balance maintained, but the animal reduced to somnolent condition. 



cerebrum, be removed, it will be found that the animal 

 still sits in its characteristic posture. When touched it 

 jumps ; when thrown into water it swims. It is a perfect 

 reflex machine, with the power of balancing itself unimpaired. 

 But it differs from a normal frog in moving only when 

 directly stimulated, and in showing no signs of hunger or of 

 thirst. A worm crawling in front of it does not cause the 

 characteristic series of movements for its capture which is 

 seen in a normal frog. Generally a condition of decere- 

 bration rigidity appears after a time, and the frog, when held 

 by the sides and placed upon the table, remains with its legs 

 rigidly stretched, supporting the body well above the surface, 

 with the back arched like that of an angry cat. 



In the pigeon (fig. 34, B), removal of the cerebral hemi- 



