338 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



Depression of the defensive ray is, of course, produced by its own 

 depressor ; but it is permitted by the action of the erector of the 

 second, which draws its ray upwards, setting it astride of its spinal 

 process, and releasing its limbs from their apposition with the fourth 

 spinous process, and so allowing of its depression. It is to be noticed 

 that the erection of the third and succeeding rays is accompanied or 

 succeeded by the contraction of the depressor of the second and 

 similarly their depression with the action of the second erector. 



The abnormal relations of these muscles can be explained by the 

 modifications of the parts. Those of the anterior ray, which is 

 almost unrecognizable and firmly fixed, are aborted. The interspinal 

 of the first ray having lost its original relations and become bent 

 upwards from its attachment to the spinous process of the third 

 vertebra until it lies longitudinally, its muscles have lost their attach- 

 ment to it, and so the erector of the second which ought to arise from 

 its posterior surface has transferred its attachment to the more solid 

 horizontal plate. The second depressor ought to arise from the 

 anterior surface of the second interspinal, but the membrane bone 

 which develops round the fourth vertebra, growing in as it were 

 between the muscle and the interspinal, separates them, and the 

 muscle passes farther forwards on the plate until it reaches the base 

 of the anterior ascending process, thereby acquiring greater obliquity 

 of action. The erectors and depressors of the third ray have in part 

 their normal relations, but owing to the weight and ossification of 

 the ray they have to move, have become enlarged, and extended their 

 origin beyond the typical limits. The erector of the fourth ray has 

 been crowded out from its original insertion by the aggression of the 

 third depressor, and has become inserted into the horizontal plate 

 where its action is more forcible. 



X.— MUSCLES OF THE ANAL FIN. 



The infracarinales act to a certain extent upon the rays of the 

 anal fin. The portion named by Owen the ' retractor ischii,' is 

 inserted posteriorly into the base of the anterior ray, the posterior 

 portion is inserted into the base of the posterior ray. Thus, when 

 these act simultaneously, or even when one acts and the other remains^ 

 fixed, the rays will be divaricated. 



