ANTERIOR MEMBERS. 55 



rays, whether branched and articulated, or simply 

 spinous, may be always divided lengthways into 

 halves. 



The vertebrae are characterised by the conical 

 hollow on each side of their faces. Double hollow 

 cones are thus formed in the interval between two 

 vertebrae, filled by a soft membranous and gelatinous 

 substance, which passes from one void to another 

 by means of an opening through each vertebra, and 

 thus forms, as it were, a gelatinous chaplet througn 

 the whole. As in the other vertebrated classes, 

 there is an annular opening through the superior 

 portion, for the passage of the spinal marrow. 



Fish rarely possess a sternum properly so called, 

 and when it does exist, it is formed of almost exter- 

 nal pieces, which unite the inferior extremities of 

 the ribs. 



The anterior members, of which the external por- 

 tion is commonly called the pectoral fin, 53 a , con- 

 sist, in the first place, of a suite of bones on each 

 side immediately behind the orifice of the gills, and 

 which form a kind of frame on which the opercle 

 rests when closed. These bones, usually attached to 

 the head above, and uniting together below, form an 

 osseous belt which almost encircles the body. Their 

 inferior symphysis unites by ligaments to the tail 

 of the hyoid bone, 42 (see cut of Cranium), and 

 forms with it the isthmus, which separates the ex- 

 ternal openings of the gills from each other beneath, 

 just as the cranium separates them above. This 

 cincture, when complete, is composed on each side 

 of three bones, which represent the shoulder and the 



