Spinal Cord. That part of the vertebrate central nervous system within the backbone. Consists of a hollow tube, but the hole is very small and the walls are relatively unevenly thick. Contains numerous nerve-cells and bundles of nerve fibres, particularly those connecting all levels of the spinal cord with the brain. Pairs of peripheral nerves, one nerve on each side per segments, leave he spinal cord to the distributed to the body.

(See Nerve Root). A great deal of simple co-ordination of movement of limbs, viscera, etc., is done in the spinal cord by reflexes.

Spinal Nerve. Peripheral nerve arising from spinal cord of vertebrate. One on each side in each segment.

Spindle.
(1) Body formed within a cell at mitosis or meriosis, taking part in distribution of chromatics to the two daughter nuclei.

Commonly but not always ellipsoid. Appears at metaphase, and chromosomes become arranged at its equator. Spindle is relatively solid (a gel). It contains many microtubule, some running.

Spinnerets. Of spiders, four to six elevation at end of abdomen, on to surface of which ducts of spinning glands open. Probably represent abdominal appendages.

Spiracle.
(1) Dorsally situated reduced first (most anterior) gill-slit im many fish. Its small size results from the connection formed between mandibular and hyoid arches for firm attachment of jaws; between these arches the spiracle lies.

(See Visceral Arch, Hyostylic Jaw-Suspension). In most living bony fish, spiracle is closed up. The gill-pouch of embryo tetrapods representing spiracle develops into cavity of middle ear and Eustachian tube.

(2) Of insects, synonym of stigma; external opening of trachea.

Splanchnocranium. Part of skull comprising jaws and their attachments.