580 



SIGNIFICANCE OF THE NERVOUS SYSTEM 



and the supra-esophageal ganglion. It appears to be made up of several and sends 

 out two commissures {B) which encircle the esophagus and eventually unite with 

 the large supra-es.ophageal ganglion. Besides, it gives origin to ten pairs of nerves 

 which innervate the mandibles, the maxillae, and the maxillipedes with their 

 branchial appendages. A still greater differentiation is pre- 

 sented by the head ganglion which consists of three pairs of 

 nodular enlargements, namely, the protocerebron, the deutero- 

 cerebron and the tritocerebron. The optic nerves (O) enter the 

 foremost of these, while the middle ones receive fibers from the 

 integument, antennae and organ of hearing (R). The posterior 

 nodules give origin to the nerves innervating the large external 

 antennae (D). 



An arrangement of this kind constitutes a typical reflex stem, 

 around which, in the higher forms, the association or reaction 

 system is developed. The greatest change is effected anteriorly 

 in the region of the head ganglia, because these bodies are destined 

 to become the recipients of the impulses from the chief sense 

 organs. Eventually, many of these impulses are not permitted 

 to pass directly upon efferent channels, but are first conducted 

 into certain complexes of nerve cells in which they are asso- 

 ciated. In this way, each sensory reflex area is finally invested 

 by a sphere of association, the nervous products of which give 

 rise to the psychic life of the animal. This development of the 

 different association centers takes place gradually. The first to 

 make its appearance is the center for smell, because smell is the 

 most primitive sense and many animals, such as the amphibia 

 and reptilia, depend upon it almost exclusively. It need 

 scarcely be emphasized that the development of these associa- 

 tion spheres increases the complexity of the nervous system 

 very pronouncedly, because the primitive reflex stem is now 

 materially augmented by the addition of the brain. Quite aside 

 from this structural and functional complexity of the nervous 

 system of the higher animal, it should be noted especially that 

 its reflex life is completely subordinated to the activities of 

 the association centers situated in. the more recently formed 

 cerebral hemispheres. 



>< 



^<^ 



4 k 



111 



Fig. 289.— 

 Diagrammatic 

 Re presentation 

 OF THE Nervous 

 System of the 

 Crayfish. 



A, Supraeso- 

 phageal gang- 

 lion; B, commis- 

 sure; C, subeso- 

 phageal ganglion ; 

 D, first abdom- 

 inal ganglion; 

 O, optic nerve ; 

 R, middle nerve; 

 P, antennary 

 nerve; S, stoma- 

 togastric nerve. 



The Joining of the Reflex Circuits. — The struc- 

 ture of the most elementary reflex arc has been fully 

 considered in one of the preceding paragraphs. It con- 

 sists of a receptor, an afferent path, a center, an 

 efferent path and an effector. Moreover, it is to be 

 noted that, in the lowest forms, these reflex circuits 

 are few in number and retain a marked independency 

 of one another. In the higher animals, on the other 

 hand, they increase greatly in number and become 

 closely linked by intermediary neurons which thus es- 

 tablish a close functional relationship between them. 

 The simple reflex arc (Fig. 290) may be compounded 

 first of all into a reflex chain consisting of several arcs 

 {B). The impulse producing the primary reflex response is thus en- 

 abled to spread and to incite other responses until the so-called chain 

 reflex is obtained. Another way in which these reflex arcs may be 

 arranged is illustrated by Fig. 290, C. Here two effectors are connected . 



