THE MYOLOGY OF AMIURUS CATUS. 337 



base of the posterior surface of the ray. The erector loses however its 

 proper origin, arising instead from posterior edge of the horizontal 

 plate on which the defensive (3rd) ray rests. 



The erector of the defensive or third ray lies in the interval be- 

 tween the second and third interspinalia. It arises from the pos- 

 terior edge of the first interspinal, the anterior edge of the second, 

 and from the posterior portion of the expanded process of the fourth 

 vertebra. It passes upwards and is inserted into the anterior surface 

 of the base of the ray. The depressor has also its origin much in- 

 creased. It arises from the sides of the third interspinal, from the 

 anterior surface of the fourth, and from the spinous process of the 

 fifth vertebra, and is inserted into the base of the anterior surface of 

 the ray. 



The horse-shoe-shaped or second ray has also an erector and de- 

 pressor. The erector is small, and consists of a few fibres, which run 

 obliquely backwards from their origin from the under surface of the 

 anterior portion of the horizontal plate, and which, passing through 

 the foramen in this plate in company with the depressor, are inserted 

 into the anterior surface of the extremity of one of the limbs of the 

 ray. The depressor is a much stouter muscle, arising from the base 

 and posterior surface of the anteriorly directed osseous process of the 

 fourth vertebra, which includes the spinous process of the third. It 

 passes upwards and backwards through the foramen in the anterior 

 portion of the horizontal plate behind the erector, and is inserted 

 into the extremity of the limb of the ray. 



The muscles of the first ray are aborted. 



Innervation. — Supplied by branches from the ramus lateralis 

 trigemini with which the R. dors, spinal, unite. 



Action. — The action of the muscles of the posterior rays are suffi- 

 ciently expressed by their names. With regard to those of the 

 second ray there is something to be said, since it is by these that the 

 fixation of the third ray is produced, and its depression permitted. 

 The depressor draws the horse-shoe-shaped ray downwards, so that it 

 slips over the smooth extremity of the interspinal, and its limbs 

 come into apposition with the flanges on the sides of the fourth 

 spinous process which encloses its interspinal. The third or defen- 

 sive ray is attached to the extremity of the second by ligament, so 

 that its depression will now be impossible. In other words, it is the 

 fixation of the second ray which causes the fixation of the third. 



