What is "pallial sinus"

Sinus \Si"nus\, n.; pl. L. Sinus, E. Sinuses. [L., a bent surface, a curve, the folds or bosom of a garment, etc., a bay. Cf. Sine, n.]

1. An opening; a hollow; a bending.

2. A bay of the sea; a recess in the shore.

3. (Anat. & Zo["o]l.) A cavity; a depression. Specifically:

   1. A cavity in a bone or other part, either closed or with a narrow opening.

   2. A dilated vessel or canal.

4. (Med.) A narrow, elongated cavity, in which pus is collected; an elongated abscess with only a small orifice.

5. (Bot.) A depression between adjoining lobes. Note: A sinus may be rounded, as in the leaf of the white oak, or acute, as in that of the red maple. Pallial sinus. (Zo["o]l.) See under Pallial. Sinus venosus. [L., venous dilatation.] (Anat.)

   1. The main part of the cavity of the right auricle of the heart in the higher vertebrates.

   2. In the lower vertebrates, a distinct chamber of the heart formed by the union of the large systematic veins and opening into the auricle.

Pallial \Pal"li*al\, a. [L. pallium a mantle. See Pall.] (Zo["o]l.) Of or pertaining to a mantle, especially to the mantle of mollusks; produced by the mantle; as, the pallial line, or impression, which marks the attachment of the mantle on the inner surface of a bivalve shell. See Illust. of Bivalve.

Pallial chamber (Zo["o]l.), the cavity inclosed by the mantle.

Pallial sinus (Zo["o]l.), an inward bending of the pallial line, near the posterior end of certain bivalve shells, to receive the siphon. See Illust. of Bivalve.

The pallial sinus is an indentation or inward bending in the pallial line on the interior of a bivalve mollusk shell's valves that corresponds to the position of the siphons in those types of clams which have siphons (i.e. siphonate). The position of the pallial sinus is often clearly visible as a shiny line on the inside of the bivalve shell.