25 



perforated centrally to form the pupil. Around the iris the choroid 

 is folded radially into the ciliary processes. 



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 dog- 

 fish 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 thor- 

 oughly 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 ex- 

 tent as to resemble soft bone. 



The parts of the skeleton are frequently grouped under two 

 heads: the axial skeleton, comprising the skull and vertebral col- 

 umn; 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 occu- 

 pying 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 com- 

 posed 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 occu- 

 pied 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. 



The concave faces of the vertebrae consist of much firmer car- 

 tilage than the remaining portions, sometimes even calcified. Make 

 a transverse section through the middle of a centrum and observe 

 the relations of the parts. 



On each side of the centrum, near the ventral edge, is a plate- 



