. Animal physiology. Physiology, Comparative; Physiology, Comparative. 336 NERVOUS SYSTEM OF ARTICTJLATA. tions; and the influence of its instincts or its will is exerted through them, upon the several parts of its body, causing spontaneous motion. By the latter are produced those reflex actions of the several organs, which do not require sensation or will, but which depend upon the simple conveyance of an impression to the gan- glion, and the transmission of a motor impulse excited in it, from it to the muscles supplied by its nerves. Of these movements in the Mollusca we know but little; in
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 336 NERVOUS SYSTEM OF ARTICTJLATA. tions; and the influence of its instincts or its will is exerted through them, upon the several parts of its body, causing spontaneous motion. By the latter are produced those reflex actions of the several organs, which do not require sensation or will, but which depend upon the simple conveyance of an impression to the gan- glion, and the transmission of a motor impulse excited in it, from it to the muscles supplied by its nerves. Of these movements in the Mollusca we know but little; in the Articulata, however, they are extremely remarkable. 440. We have seen that, in the Mollusca, a small part only of the Nervous System ministers to the general movements of the body; and this corresponds with what has been elsewhere stated (§. 75) of the inertness which is their general character- istic, and of the small amount of muscular structure which they possess. On the other hand, in the Articulated classes, in which the apparatus of movement is so highly developed, and whose actions are so energetic, we find the Nervous System almost entirely subservient to this function. Its usual form has been already described (§. 70 as a chain of ganglia con- nected by a double cord, which commences in the head, and passes backwards through the body. In general, we find a ganglion (or rather a pair of ganglia united on the middle line) in each segment; hence in the Annelida and Myriapoda, the ganglia are very nu- merous ; but they are proportionably small. In Insects, the number of segments, and consequently of ganglia, never exceeds thirteen; and the ganglia are larger.—Whatever be the number of the ganglia, they are usually but repetitions of one another, the functions of each segment being the same with Fig. 173.—Nervous System of an . .. , _. ° . Insect. those of the rest. I he nerves proceed- ing from them are chiefly distributed to the muscles of the legs; or, where t
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