. A manual of zoology. Zoology. 86 GEXERAL TRLXCirLES OF ZOOLOGY sj'stem. In the ganglia (the nervous eentres of the kist three groups) the ganglion cell usually gives rise to a single strong process, which, how- ever, is richly provided with lateral branches or dendrites (tig. 77). The medullary sheath and sheath of Schwann are usually absent in inverte- brates even in the peripheral nerves. On the other hand, the true con- ducting elements, the nerve librilkv, have been seeii in invertebrate nerve- tibres, and have been followed into the ganglion cell in which the atierent and efferent fibri
. A manual of zoology. Zoology. 86 GEXERAL TRLXCirLES OF ZOOLOGY sj'stem. In the ganglia (the nervous eentres of the kist three groups) the ganglion cell usually gives rise to a single strong process, which, how- ever, is richly provided with lateral branches or dendrites (tig. 77). The medullary sheath and sheath of Schwann are usually absent in inverte- brates even in the peripheral nerves. On the other hand, the true con- ducting elements, the nerve librilkv, have been seeii in invertebrate nerve- tibres, and have been followed into the ganglion cell in which the atierent and efferent fibrilkc are united in a lattice-like s ot an acunian. A ganglion cell with its dendrites and neurites (the latter in large animals may be several feet long, since they extend from the central nervous svstem to the muscles) form a phvsiological unit, called a neurone. The advocates of the 'neurone theory' maintain that the processes of two neurones do not anastomose and that there is no continuity between them; they approach each other so closely that the nervous impulse may jump from one to another like an electric spark. The opponents of the theory assert that there is real continuity as seems to be certainlv shown with the giant ganglion cells of the nematodes. A second disputed point is that the neurone theory holds that all processes, even the long- est neurite, grows from the ganglion cell and throughout its length is a product of it. The opponents claim that the neurites which compose the peripheral ner\e fibres are formed by special nerve-forming cells {neuroblasts) and these contribute to the length of the neurite starting from a ganglion cell. In many invertebrates (Hirudinei) such neuroblasts persist in the course of the peripheral nerves. Also the peripheral system of invertebrates can consist of a network of anastomosing neuroblasts (gang ionic plexus of medusa; and eha>lognaths). When we recall that the primitive eoelenterates (hydroid polv]is) lack a centr
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
