COORDINATION 209 



bral column, from which branches proceed to innervate the nearby 

 organs. It communicates with the central system by way of the 

 sensory roots of the spinal and some of the cranial nerves. 

 (Figs. 142, 144.) 



Such, in essence, is the distribution 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 isolated 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, known as internal receptors, 

 are situated within the body for the reception of stimuli arising 

 there. The external 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 or- 

 gans. Nevertheless all forms, in common with all protoplasm, 

 possess the power of receiving and responding to environmental 

 changes. Thus Paramecium reacts to mechanical, thermal, chem- 

 ical, and electrical stimulation: the entire surface of the cell is 

 sensitive to stimuli, and the excitations are conducted from one 

 part to another essentially by the protoplasm as a whole, aided 

 apparently by the neuromotor apparatus. In some Invertebrates, 

 such as Hydra and the Earthworm, the entire surface of the body 

 is still depended upon as a receiving organ for all kinds of stimuli, 

 and only simple sense cells are developed. In the majority of ani- 

 mals, however, although all the cells, of course, retain to some ex- 

 tent their power of irritability, environmental changes exert their 

 influence chiefly upon complex receptors which are specialized to 

 respond most readily to particular forms of energy. The energy, 

 for example of heat or light, is transformed by appropriate 

 mechanisms into the energy of a nerve impulse, and accord- 

 ingly the sense organs constitute the outposts of the nervous 

 system. (Figs. 22, 135-137, 139, 225.) 



Since we necessarily gain our knowledge of the outside world 

 solely through the data afforded by our sense organs, it follows 



