324 



THE VERTEBRATE BODY 



those going from the spinal cord or brain lead to muscles and are known 

 as motor nerves. 



A simple nerve stimulation and motor reaction can be illustrated as 

 follows. If one of the sense organs of touch on the foot were stimu- 

 lated by pricking it gently with a pin, an impulse would be generated 

 that would travel up the sensory nerve and enter the spinal cord. It 



Ciliary body 



Retina 



Conjunctiva 



Choroid 



Optic nerve 



Blind spot 



• Anterior chamber with aqueous humor 



Elements of Biology, Strausbaugh and Weimer, John Wiley & Sons, Inc. 



Fig. 22.15. Section through a human eye showing the parts found in a typical chordate 



eye. 



would continue up one of its nerves within the spinal cord to the brain 

 and the frog would know that it had been touched. Connections would 

 be made and the impulse would travel back down one of the nerves 

 within the spinal cord, out into a motor nerve, and arrive at one of the 

 leg muscles, which would contract and move the foot from the pin. Such 

 is the chain of events that regulates the thousands of adjustments that 

 higher animals make to their surroundings. There is a slight delay in 

 the stimulation and response, for it takes some time for the impulses to 



