THE FUNCTIONAL UNIT OF THE NERVOUS SYSTEM 



579 



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E* 



fibers extend in all directions to the different tissues of the segment. 

 A stimulus applied to its surface (S), is soon followed by movements or 

 some other motor response (E) , and hence, we must conclude that the 

 nervous material allotted to each segment, is arranged in the form 

 of reflex arcs, the centers of which are situated in the ganglion. While 

 a high degree of independency is thus assured to each segment, it must 

 be admitted that the life of the entire animal requires in addition a 

 proper correlation between its different 

 parts. The functions of its segments must 

 be subordinated to the requirements of the 

 whole. This end is accomplished by in- 

 termediary neurons (A) which unite the 

 successive ganglia with one another. These 

 association fibers are placed longitudinally 

 to the long axis of the animal and form in 

 this way a conducting channel akin to the 

 spinal cord of the higher forms. It is also 

 to be noted that the head ganglia are es- 

 pecially well developed and exercise a con- 

 trolling influence over the other ganglia. 

 Eventually these central complexes also 

 become the recipients of impulses from cer- 

 tain sense-organs, such as the eyes, and the 

 receptors for chemical and vibratory im- 

 pacts. This is of importance, because the 

 movements and general behavior of the 

 animal naturally demand a proper correla- 

 tion of all these different sensory impres- 

 sions. It should be emphasized, however, 

 that those animals which are equipped 

 with a nervous system of this kind, are not 

 capable of forming associations. They are, 

 therefore, true reflex animals for the reason 

 that they are not in possession of those 

 complexes of neurons which give rise to 

 psychic activities (cerebrum). 



H 



S 1 



FIG. 288. DIAGRAMMATIC 

 REPRESENTATION OF THE NER- 

 VOUS SYSTEM OF A SEGMENTAL. 

 ANIMAL. 



G l , G* and G 3 , Ganglia in 

 three successive segments. 

 S 1 , S 2 and S 3 , and E 1 , E 2 and 

 E 3 , the receptors and effectors 

 of those segments forming the 

 end stations of typical reflex 

 circuits; A, association paths 

 uniting the reflex centers of 

 the successive segments. 



By way of illustration, let us devote a few 

 moments to a consideration of the nervous system 

 of the crayfish (Fig. 289). It consists of thirteen 



ganglia, six of which are allotted to the abdomen, six to the thorax and one to the 

 head region. As the most anterior of these lies in close relation with the esophagus, 

 it is usually called the supra -esophageal or supramaxillary ganglion (A). The fibers 

 emitted by every one of these ganglia, are distributed to the muscles and the sense- 

 organs of the integument. They are brought into relation with those of the opposite 

 side by intermediate neurons. Connections are also made with the neighboring 

 ganglia by means of commissural fibers. Each ganglion, therefore, is partially 

 divided into two lobes and this bilateralism is also apparent in the path connecting 

 them. The first thoracic or subesophageal ganglion (C) is more highly developed 

 than the others, because it forms the link between the chain of posterior ganglia 



