Physiology of some Zoophytes. 395 



for receiving the hinge-process; and the smallest is placed at the 

 lower end^ and aiFords a passage to the posterior part of the 

 pedicle into the interior of the body. The hinge-process is con- 

 cave on its upper surface, and terminates below in a curved point. 

 Its superior wall forming its concave surface is deficient in two- 

 thirds of its length at the upper part or that next the articulation, 

 and its inferior or convex wall is very thin over the same extent. 

 It is hollow, and communicates with the body through the upper 

 and middle apertures seen in its concave or upper surface. Its 

 upper or articulating end is bounded by a thickened portion or bar 

 passing between the edges of the superior surface, and a similar 

 bar passing between the edges of the inferior or convex surface. 

 The articulating processes are placed upon the superior of these 

 bars, at its junction with the edges of the superior surface. I have 

 described, with what may appear very unnecessary minuteness, 

 the skeleton of these bird-head processes, because it would be im- 

 possible to understand their movements without a previous know- 

 ledge of the different parts described. The lateral portions of the 

 lower chamber of the body are occupied by two radiating muscles, 

 presenting somewhat of the appearance of the temporal muscle in 

 the human species, which converge at the articulating or upper 

 edge of the hinge-process, and terminating in a denser, thicker 

 and narrower structure, which I shall call tendons, are attached 

 to and move this process (fig. 12 c). One of these muscles, which 

 is the stronger, terminates in a tendon which runs above the 

 transverse bar which separates the upper from the middle aperture 

 in the concave surface, and running down the centre of the hinge- 

 process is inserted into the inner surface of its inferior or convex 

 wall a little above its apex or free extremity. When this muscle 

 contracts, the hinge-process is tilted up. The other muscular 

 bundle, which is strongest at the upper and lower edges, terminates 

 in a tendon which passes beneath the bar, and is inserted into 

 the hinge-process close to and a little above the tendon of the 

 other muscle. When this muscle contracts, the hinge-process, 

 if elevated, is drawn down. The first-described muscle is the 

 elevator J the second is the depressor muscle of the hinge-process. 

 The movements of the hinge-process are in general slight, but I 

 have frequently observed it to be tilted up with considerable 

 force, and closely applied over the superior surface of the anterior 

 chamber, so that its concave, which was before its superior, be- 

 came its inferior surface, and its convex became its superior sur- 

 face. In this state it may remain for hours, and affords an ex- 

 cellent opportunity for observing the arrangement of the fibres 

 of the two muscles, especially that of the elevator, as its lower 

 fibres run more directly upwards, and its tendon is raised and 

 separated from that of the depressor muscle. In dead specimens 



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