468 



COMPARATIVE PHYSIOLOGY. 



(3) its character, and (4) the condition of the spinal cord at the 

 time. Occasionally on irritating one fore-Umb the opposite 

 hind one answers reflexly. Such is a '' crossed reflex," and is 

 the more readily induced in animals the natural gait of which 

 involves the use of one fore-leg and the opposite hind-limb 

 together. 



■ ff3JfCB 



Fie. 333.— Diagram of a spinal segment allowing component parts (Ranney). 



Fig. 334.— Diagrammatic representation to lllnstrate the reflex arc (Bramwell and Ban- 

 ney). 1, 2, sensory fibers; 3, motor-cell of anterior horn; 4, motor-flber connected 

 witli 3 and passing out by anterior root to muscle; 5, fiber joining ganglionic cell 

 (3) with crossed pyramidal tract, C. P. C; 6, ganglion on root of posterior spinal 

 nerve; 7, fiber joining 3 with Tfirck's column, T. Fiber 2 is represented as pass- 

 ing through Burdach^s column to reach the cell. 3. 



Eeflexes are often spoken of as purposive, and suggest at 

 first intelligence in the cord; but such phenomena are explained 

 readily enough without such a strained assumption. 



Evolution, heredity, and the law of habit, apply here as else- 

 where. The relations of an animal to its environment must 

 necessarily call into play certain nervo-muscular mechanisms, 



