is behind it. The orbital artery is absent in Typhlonarke. This fish is blind, and the eyes are reduced to small remnants, but as the orbital artery usually supplies a considerable part of the anterior region of the head as well as the eye muscles, this would not seem to account for its absence. In Torpedo, R. nasuta and the Stingray, the hyoidean epibranchial artery gives rise to an orbital artery (o.a.) which passes ventral to the brain case, and in the orbit runs dorsal to the efferent spiracular artery (e.s.a.) but ventral to nerves V and VII, to which it runs parallel. In front of the eye it divides into two, the median branch supplying the ventral surface of the rostral cartilages and giving a branch to the roof of the mouth, the lateral one turning posteriorly to supply the muscles of the upper jaw.

After the orbital artery has been given off, the two vessels, now usually known as the internal carotid arteries and representing morphologically the anterior ends of the lateral dorsal aortae, proceed anteriorly and unite in the middle line to pass through a foramen on the floor of the brain case. Inside, a branch is given dorsally which divides into anterior and posterior cerebral arteries. The posterior cerebral arteries pass along the posterior region of the brain to the spinal cord and are there augmented by branches of the segmental arteries.