. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. SENSE-ORGANS OF THE INTEGUMENT 191 MZ^.. a level with the epidermis/ but in Fishes they may in post-em- bryonic time become enclosed in depressions or complete canals,- which are formed either by the epider- mis only, or, as is more usually the case, by the scales and bones of the head, and which open externally. The organs are thus protected. These sensory organs are situated characteristically along certain tracts, the position of which is very constant: in the head, supra-orbital, infra-orbital, and hyomajidibular t
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. SENSE-ORGANS OF THE INTEGUMENT 191 MZ^.. a level with the epidermis/ but in Fishes they may in post-em- bryonic time become enclosed in depressions or complete canals,- which are formed either by the epider- mis only, or, as is more usually the case, by the scales and bones of the head, and which open externally. The organs are thus protected. These sensory organs are situated characteristically along certain tracts, the position of which is very constant: in the head, supra-orbital, infra-orbital, and hyomajidibular tracts can be recog- nised, and a lateral line (or several— Proteus and all Amphibian larv^) ex- tends along the sides of the body to the caudal fin (Figs. 152 and 153). They are thus often spoken of as segmental sensory organs or organs of the lateral line} The portions lying in the region of the head are innervated by the lateral line branches of the facial, glossopharyngeal, and vagus {see note on p. 185). Freely projecting nerve-eminences are not present in Eays and Ganoids, and are only of minor importance in Sharks. In all these Fishes the integumentary sense-organs are more or less deeply situated, being enclosed in complete or incomplete canals arising as proliferations of the epidermis extending into the dermis, and becoming greatly branched. The so-called Savi's vesicles of Torpedo, the '"nerve sacs" of Ganoids, and the ampulkv of Elasmobranchs, correspond to modified nerve-eminences. They are all limited in their distribution to the head and anterior portion of the trunk, being most numerous on the snout: they arise from thickenings of the epidermis which later become invaginated and in which a sensory epithelium is differ- entiated. In Ganoids these organs retain a simple sac-like form, and in Torpedo they become completely separated off from the epidermis, while in other Elasmobranchs they are tubular, each tube giving rise to one or more swellings or am
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
