341! PROCEEDINGS OF THE CANADIAN INSTITUTE. 



would persist to a greater or less extent. And so indeed they have 

 done. In the Teleosts there are as representatives of the constrictors, 

 the inter mandibularis, the add. and lev. arc. pal., lev. and add. opere., 

 the transversi dorsales and ventrales of the branchial arches, the 

 interarcuales veutrales, etc. In these muscles the course of the fibres 

 is parallel to a plane at right angles to the axis of the body, and 

 they act more or less as constrictors of the parts to which they are 

 attached. The greater mass of the constrictors of the Selachians is 

 in relation to the branchial cavity. Where the parts about the 

 pharynx are comparatively elastic, constrictor muscles will be, of 

 course, of great use in diminishing that cavity, and so forcing the 

 water out through the gills ; but when, on the other hand, the parts 

 become less movable through ossification, other arrangements for the 

 propulsion of the water appear. Membrane bones are developed to 

 act as valves and pi-otections to the gills, a portion of the constrictor 

 musculature persisting, attached to them, and the lessening of the 

 size of the pharyngeal cavity is produced by the elevation of certain 

 parts in the floor of the mouth, and only slightly by the approxima- 

 tion of the walls by constrictors. These latter, therefore, become 

 limited to certain parts, instead of forming a more or less unbroken 

 sheet over the branchial region. 



Bearing in mind the fact that in the head there were originally a 

 number of myomeres, as represented by the head-cavities, which 

 have been specialized into a number of distinct muscles ; and that 

 to a very large extent the muscle fibres have lost their original 

 direction, it is possible by means of the innervation to refer to their 

 respective myomeres the various muscles. 



The Cranial Muscles. — Leaving out of consideration the muscles 

 of the eyeball, which belong to a myomere or myomeres in front of 

 the mouth, the first muscle segment to be considered will be that 

 supplied by the fifth nerve. Belonging to this there is, in the first 

 place, the add. mand., the fibres of which have, to a large extent, a 

 longitudinal direction, and which extends between the mandibular 

 and hyoid arches. Reasoning from analogy one would have ex- 

 pected to find this muscle and those belonging to the same myomere 

 extending between the first prseoral and the mandibular arches, but 

 we find them in reality lying superficially to certain muscles sup- 

 plied by the facial nerve. The development of the first prseoral (or 

 palatine) arch being in comparison with the succeeding ones so 



