496 GENERAL CONSIDERATIONS 



of the neuromuscular cells in the sponges, which are thus independent 

 effectors. There are also muscle cells in higher animals which are inde- 

 pendent effectors, responding directly to a stimulus. Such are the cells 

 of the circular muscles of the iris the response of which is due to the 

 presence of a light-sensitive pigment in the cells. 



In the coelenterates have been noted simple receptor-effector mecha- 

 nisms and the presence of a nerve net. The activity of these structures 

 leads to responses which are nonspecific — that is, they do not differ with 

 different stimuli. The responses follow no definite path, and the struc- 

 tures are autonomic — that is, independent, functioning when severed 

 from the rest of the body. Such a mechanism does not show polarity, 

 which is the property of transmitting impulses only in one direction, this 

 being a property of a synaptic system made up of neurons. 



The next step in the phylogenetic development of the nervous system 

 is the development of ganglia, which results in the appearance of the 

 receptor-adjustor-effector mechanism, or the reflex arc. Owing to the 

 polarity which exists in neurons the character of any reflex act is predict- 

 able. At the same time there appear what are known as conditioned 

 reflexes the result of which depends not only upon the appropriate stimulus 

 but also upon a cerebral element, higher centers in the central nervous 

 system either favoring or inhibiting the carrying out of the reflex act. At 

 first the ganglia in different parts of the body are to a large degree inde- 

 pendent of one another and only at certain times is their activity coordi- 

 nated. As the nervous system becomes more highly developed in higher 

 forms centralization appears, and then cephalization. Centralization and 

 cephalization reach their highest development in the vertebrates, cul- 

 minating in man. 



Although centralization and cephalization have been carried to the 

 highest extent in man, there still remain scattered over the human body 

 a large number of centers controlling small groups of organs and govern- 

 ing certain acts which may take place independently of the will. Many 

 of these acts, however, can with sufficient warning be controlled. Such 

 centers, which are most numerous in the medulla, govern respiration, 

 steady the beating of the heart, and control mastication, swallowing, 

 sucking, the reflex secretion of the sahva and digestive juices, vomiting, 

 coughing, sneezing, and winking. Numerous other similar centers are 

 scattered up and down the cord and exist in outlying ganglia in the cere- 

 bral, spinal, and sympathetic nervous systems. 



Sense organs, generally speaking, are known as receptors, but sensa- 

 tions are functions not of sense organs but of the central nervous system. 

 The appropriate receptor, for instance, receives a chemical stimulus and 

 sends an impulse to one or the other of two centers in the brain, which 

 when stimulated gives rise to the sensation of taste in the case of one or 

 of smell in the case of the other. • 



