Propulsion is by means of the dorsal fin (that is, the large fin arising from the midline of the back).
The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber.
The thymus is divided into two lobes, lying on either side of the midline of the body, and into smaller subdivisions called lobules.
The pineal gland develops from the roof of the diencephalon, a section of the brain, and is located behind the third cerebral ventricle in the brain midline (between the two cerebral hemispheres).
Some fibres from the ventral cochlear nucleus pass across the midline to the cells of the superior olivary complex, whereas others make connection with the olivary cells of the same side.
In both pathways some of the fibres remain on the same side, while others cross the midline to the opposite side of the brain.
The flaring upper part of the girdle is the ilium; the lower anterior part, meeting with its fellow at the midline, is the pubis; and the lower posterior part is the ischium.
An imaginary line drawn down the midline of the ring finger represents the junction of the ulnar-radial distribution on the back of the hand and the ulnar-median distribution on the palm.
A midline lesion of the medulla oblongata is likely to involve the pyramidal tracts (the descending motor pathway) and the medial lemnisci (the ascending tracts relaying sensory impulses from the dorsal columns of the spinal cord).
Damage to the oldest part of the cerebellum, which lies deep in the midline, results in difficulty in maintaining an upright posture.
Some vertebrate species have only one gonad, which may lie in the midline or on one side; the condition is more common among females.
