vii. 4-5 BACKBONE OF FISHES 199 



area of the respiratory surface is thus an important limiting factor in 

 the movement and growth of fishes. During activity of a fish lactic 

 acid accumulates in the blood and the pH falls. The fish is thus able 

 to display a considerable burst of activity and then to repay the 

 oxygen debt over a long subsequent period. 



5. Vertebral column and fins of bony fishes 



The vertebral column of bony fishes performs the same function as 

 in other fishes, namely, to prevent shortening of the body when the 

 longitudinal muscles contract. It has, however, become very compli- 

 cated and with the ribs and neural and haemal arches forms an 

 elaborate system serving to maintain the body form under the stresses 

 of fast swimming. Like other parts of the skeleton it is extensively 

 ossified, and the necessary lateral flexion is obtained by division of 

 the column into a series of sections joined together. Typically there 

 is one such section (vertebra) corresponding to each segment, but in 

 the tail region of Amia there are twice as many vertebrae as segments. 



Each vertebra consists of a centrum, neural arch and neural spine, 

 and in the tail region, in addition, haemal arch and haemal spine. 

 These parts are formed partly by ossification of cartilaginous masses, 

 the basidorsal and basiventral, interdorsal and interventral, such as 

 we saw in elasmobranchs, and partly by extra ossification in the sclero- 

 genous tissue around the notochord and nerve-cord and between the 

 muscles. The vertebrae are inter-segmental, the middle of each lying 

 opposite the myocomma that separates two muscle segments. 



The centra are concave both in front and behind (amphicoelous), 

 and in the hollows between them are pads made of the remains of the 

 notochord, an arrangement that allows the column to resist longi- 

 tudinal compression and yet remain flexible; similar flat or concave 

 articulations of the centra are found in other aquatic vertebrates from 

 the elasmobranchs to the whales. Extra processes on the front and 

 back of the vertebrae ensure the articulation and are comparable to the 

 zygapophyses found in tetrapods. The ribs, which are so prominent 

 in the backbone of many fishes, are of two sorts; pleural ribs between 

 the muscles and the lining of the abdominal cavity, and more dorsal 

 intramuscular ribs. Both sorts are attached to the centrum. The bony 

 rods attached above the neural and below the haemal arches are often 

 called neural and haemal spines, though it is doubtful whether they 

 correspond to the neural spines of land vertebrates. They form the 

 supporting rods or radials of the median fins and are usually divided 

 into two or three separate bones in each segment. In addition to these 



