COORDINATION IN ANIMALS 



193 



Such in essence are the ramifications throughout the body 

 of the nervous system which, although it arises as an infolding 

 of the ectoderm and therefore is primarily external, comes to 

 be internal and so chiefly dependent upon more or less iso- 

 lated groups of sensory cells for the reception of stimuli. 

 Some of these, termed EXTERNAL RECEPTORS, remain at the 

 surface to receive stimuli from the outer world, while others, 



escort 



FIG. 107. Diagram of a section of the spinal cord to show the paths of nerve im- 

 pulses, c.c, central canal; col, collateral fibers; c. corl, cells of the cortex of the cerebral 

 hemispheres of the brain; c.g, smaller cerebral cells; d.c, cells in dorsal part of gray 

 matter; d.r, dorsal root of spinal nerve; g, ganglion of dorsal root; g.c, cell body of 

 sensory neuron; g.m, gray matter of cord; M, muscle; m. c, nerve cell in medulla; TO./, fi- 

 ber (axon) of motor neuron ; s, sensory surface ; s.f, fiber of sensory neuron ; spc, spinal cord ; 

 c.c, cells in ventral part of gray matter; v.r, ventral root of spinal nerve; w.m, white 

 matter of cord. The arrows indicate the direction of the impulses. (After Parker and 

 Parker.) 



known as INTERNAL RECEPTORS, are situated within the 

 body for the reception of stimuli arising there. The ex- 

 ternal receptors are what one ordinarily thinks of as sense 



organs. 



C. SENSE ORGANS 



Although among some of the Protozoa certain regions of 

 the cell are specialized so that they are more sensitive to 

 one or another kind of stimulation, the great majority show 

 no trace of sense organs. Nevertheless all forms, in common 

 with all protoplasm, possess the power of receiving and re- 



