The anatomy of the nervous system, from the standpoint of development and function . SYSTEM The sensory cells are not so directly connected with muscle-fibers in theworms as in the sea-anemones, for between receptor and effector there is hereinterposed a central nervous system. This system, as it appears in the earth-worm, is illustrated in Fig. 2. It consists of a cerebral ganglion dorsal to thebuccal cavity and a row of ventrally placed ganglia bound together by a ventralnerve cord. The most anterior of the ventral series of ganglia is connected tothe dorsal one by nerve strands on either si
The anatomy of the nervous system, from the standpoint of development and function . SYSTEM The sensory cells are not so directly connected with muscle-fibers in theworms as in the sea-anemones, for between receptor and effector there is hereinterposed a central nervous system. This system, as it appears in the earth-worm, is illustrated in Fig. 2. It consists of a cerebral ganglion dorsal to thebuccal cavity and a row of ventrally placed ganglia bound together by a ventralnerve cord. The most anterior of the ventral series of ganglia is connected tothe dorsal one by nerve strands on either side of the esophagus. The gangliaof the ventral cord are placed so that one occurs in each body segment, andfrom each three pairs of nerves run to the skin and muscles of that arrangement of the constituent elements can best be studied in transversesections (Fig. 3). The sensory cells are located in the skin, and from each ofthem a fiber runs along one of the nerves into the ganglion, within which itbranches, helping to form a network known as the neuropil. Within each. Fig. 3.—Transverse section of the ventral chain and surrounding structures of an earthworm:cm, Circular muscles; ep, epidermis; tin, longitudinal muscles; mc, motor cell-body; mf, motornerve-fiber; sc, sensory cell-body; sf, sensory nerve-fiber; vg, ventral ganglion. (Parker.) ganglion are found large nerve-cells from which fibers run through the nervesto the segmental musculature. Here we have the necessary parts for the sim-plest reflex arc. Stimulation of the sensory cell causes nerve impulses to travelthrough its fiber to the neuropil, thence to a motor cell, and finally along a proc-ess of the latter to the muscle. In other words, we have a receptor, conductor,center, another conductor, and finally an effector; and all this is for the purposeof bringing the muscle-fiber under the influence of such environmental changesas are able to stimulate the sensitive receptor. In addition to the primary s
Size: 2461px × 1015px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjectnervoussystem, bookye
