68 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



skeletogenous septum. Additional ribs may also be present, articulated with 

 centrum or neural arch and extending out into the myosepta between myotomes. 

 It appears that in teleosts ribs may be formed at almost any level of the 

 myosepta. Make a diagram showing the vertebra and its ribs. 



5. The vertebral column as a whole.— Study the entire mounted skeleton 

 of a bony fish. Note that the axis and strongest part of the column consists of 

 the centra, which are jointed together to form a longitudinal series. The 

 vertebral column is divisible into two regions, a trunk region and a tail region. 

 In the former the haemal arches are reduced to stumps, the transverse processes, 

 to the ends of which the long slender ribs are articulated. In the tail region 

 ribs are lacking and haemal arches are present. Observe the transition between 

 trunk and tail regions. In the posterior part of the trunk there is a gradual 

 elongation of the transverse process and a reduction of the ribs. At the beginning 

 of the tail region the reduced ribs finally vanish, and the elongated transverse 

 processes fuse to form the haemal arch. Note that the neural canals of successive 

 vertebrae together form a continuous canal which in life contains the spinal 

 cord. Similarly the haemal canals of the tail region form a continuous cavity 

 inclosing in life blood vessels. At the end of the tail observe the enlargement 

 and flattening of the haemal arches forming what are called hypural bones, which 

 support the tail fin. The last centrum (probably consisting of several centra) 

 forms an elongated slender bone (the urostyle) which turns sharply upward, 

 producing an asymmetry; but the symmetry of the tail is restored by the arrange- 

 ment of the hypural bones. Tails of this kind are called homocercal. 



F. VERTEBRAL COLUMN OF AMPHIBIA 



i. The vertebral column of urodeles. — Whole skeletons of urodeles such 

 as Necturus and Cryptobranchus are needed for this study. With such a skeleton 

 before you note the parts of the vertebrae of different regions. The tail vertebrae 

 are similar to the tail vertebrae of fishes. Each possesses a low neural arch, a 

 haemal arch, and a centrum bearing transverse processes. 1 In the trunk region 

 the haemal arch is absent, and the transverse processes bear ribs, separated from 

 them by a suture. The vertebrae are articulated to each other by processes 

 known as zygapophyses. These consist of a projection on each side of the neural 

 spine which fits over a similar projection arising from the anterior end of the 

 succeeding vertebra. Thus, each vertebra has a pair of prczygapophyses on its 

 anterior end whose articulating surfaces face upward, and a pair of post- 

 zygapophyses on its posterior end whose articular surfaces face downward. 

 These zygapophyses yoke the vertebrae together. 2 



1 In fishes the transverse processes were stated to be the stumps of the haemal arches. It seems 

 probable that the transverse processes on the vertebrae of land vertebrates are not of this kind but are 

 lateral outgrowths of the centrum. 



2 Zygapophyses are poorly developed or absent in fishes. 



