PHYSIOLOGY 181 



effects of previous stimuli. In the final analysis, the perceptions 

 and reactions of man are but expressions of these primitive func- 

 tions in a more specialized organism. 



The protozoan organism has no special nervous system but de- 

 pends on the primitive properties of irritability and conductivity 

 to guide its activities. In the simpler Metazoa, such as the coelenter- 

 ates, there are scattered nerve cells connected with each other by 

 fibers to form a nerve net. The neuroepithelial or neuromuscular 

 cells which make up this continuous net through the body are the 

 forerunners of the typical neurone and are called protoneurones by 

 Parker. A protoneurone transmits in every direction while a true 

 neurone transmits in only one. In the net system there is no central 

 exchange and no specific path of conduction. Every part of the 

 receptor surface of such an organism is in physiological continuity 

 with every other part of the body. 



Next comes the linear type of nervous system in the form of a 

 ladder. It is composed of an organization of neurones into a double 

 chain of ganglia, each cord lying lateral to the digestive tract with 

 transverse connectives and predominant ganglia at the anterior end. 

 Such a system will be studied in Planaria. In Annelida and Arthro- 

 poda the nervous system is a modified ladder type in which the two 

 longitudinal cords of ganglia have fused along most of the midventral 

 line. Toward the anterior end, the cords separate at a paired gan- 

 glionic enlargement, the sub esophageal ganglion, arid encircle the ali- 

 mentary canal to join on the dorsal side as the pair of supraesophngeal 

 ganglia or ''brain." In Arthropoda the ganglia of the thorax have 

 undergone considerable fusion. In Echinodermata, the starfish for 

 example, the central group of ganglia makes up the circumoral nerve 

 ring around the mouth, and radial branches extend into each arm. 

 Branches from these communicate with the sensory structures of the 

 skin and tube feet. 



Concentration of the tissue of the nervous system into definite 

 organs is carried farther in vertebrates than in the less highly or- 

 ganized forms. The fact that the central nervous system of verte- 

 brates is dorsally located and hollow has been brought out previously. 

 Even within the group of vertebrates, the nervous system shows a 

 progressive increase in complexity. The highly developed brain of 

 the mammal is the climax of this tendency. 



The neurones have been referred to before as the units of structure 

 and function of the higher types of nervous system, from worms to 



