The lens, a spherical body, transparent and elastic during life, but opaque and hard in preserved specimens. It projects into the pupil and is suspended from the ciliary processes by a delicate membrane, the suspensory ligament.
The anterior chamber, in front of the iris and lens, filled with a watery fluid, the aqueous humor.
The transparent cornea, forming the outer side of the eyeball, continuous with the sclerotic.
Take out the other eye and cut it in two by a section through the pupil and optic nerve. Review the relation of the parts.
THE SKELETON
There seems to be no easy way of cleaning the skeleton of dogfish which have been preserved in formalin or alcohol, the only procedure being to cut, pick, and scrape the flesh away from the skeleton. Time and patience are required, but if these are allowed there is no reason why all the parts of the skeleton cannot be thoroughly studied. Specimens which have been preserved in brine are more easily skeletonized.
The skeleton is entirely composed of cartilage which, in large species of elasmobranchs and in old individuals of small species, becomes impregnated with lime salts, in some cases to such an extent as to resemble soft bone.
The parts of the skeleton are frequently grouped under two heads: the axial skeleton, comprising the skull and vertebral column; and the appendicular skeleton, including the pectoral and pelvic girdles and the skeleton of the fins.
Vertebral column. The vertebral column is divided into two regions, thoracic and caudal, distinguished by the slightly different character of the vertebrae. Remove the muscle and connective tissue from the vertebral column for a short distance anterior to the first dorsal fin. Care is required not to cut away small cartilages occupying the positions of ribs. Now remove from the body about two inches of the portion of the column exposed with any cartilaginous parts which may be attached to the vertebrae. The vertical column is made up of segments, called vertebrae. Each vertebra consists of a large ventral mass, the centrum, and an arch, the neural arch, roofing over the dorsal surface of the centrum; the arch is composed of several small plates of cartilage. The opening enclosed by each centrum and its neutral arch is the vertebral foramen; the joined vertebral foramina form the neural canal, which is occupied by the spinal cord.
Separate one of the vertebrae from the rest. The centrum is deeply concave at each end; such a centrum is termed amphicoelous. At the middle of the centrum the concavities meet and thus a canal is formed through it. This canal and the spaces between the ends of adjoining vertebrae are filled by the remains of the notochord, a rather pulpy structure extending from end to end of the vertebral column.