Typically, the lateral line system consists of certain canals or grooves, usually but not invariably continuous, and defined by their innervation, (i.) a _lateral canal_ extending along the side of the body and the hinder part of the head, and having its sensory organs supplied by the great lateralis nerve (Fig. 220); (ii.) a _supra-orbital canal_ passing forwards over the eye and innervated by the superficial ophthalmic branch of the facial nerve; (iii.) an _infra-orbital canal_ supplied by the buccalis and otic branches of the same nerve; and (iv.) a _hyo-mandibular_ or _operculo-mandibular canal_, situated on the outer side of the hyoid region, and thence prolonged downward and forward in relation with the lower jaw, and innervated by the external mandibular branches of the facial nerve. The hyo-mandibular canal is sometimes distinct from the other canals, as in Elasmobranchs and some Teleosts (Fig. 220); and in certain North American Siluroids the same may be said of the supra-orbital. But, as a rule, the infra-orbital is continuous behind both with the lateral and the supra-orbital canals, while the hyo-mandibular canal joins the infra-orbital, or, exceptionally, {386}the supra-orbital canal. Transverse commissural canals often connect the lateral and supra-orbital canals of opposite sides across the dorsal surface of the head, and the corresponding infra-orbital and hyo-mandibular canals may also be continuous at the extremity of the snout or at the mandibular symphysis. [Illustration: FIG. 220.—Sensory canals of the left side of the head of _Gadus virens_. _e_, Eye; _i.o_, infra-orbital canal (dotted); _l.c_, lateral canal (oblique shading); _n_, nasal apertures; _op_, operculum; _op.m_, operculo-mandibular canal (longitudinal shading); _p.o_, pit-organs; _s.o_, supra-orbital canal (cross-hatched); _s.o.c_, supra-orbital commissure; _s.t_, supra-temporal branch; _t.t_, tubuli by which the canals communicate with the exterior. (From Cole.)]