714 DR. W. B. CAEPENTEE ON THE 8TRUCTIJRE, PHrsiOLOGT, AND 



are the deeper ; and they afford a surface of considerable extent for the attachment of 

 the two great flexor muscles of the Hay, which fill up the whole space between the upper 

 transverse ridge and the convex margins of the vertical plates, having an attachment also 

 to the prolongations d, d of those plates, which stand forth as ridges bounding the median 

 fun-ow. The shallower fossa? {b, h) give attachment to interarticular ligaments, of which 

 the special function seems to be to hold together the plates ; since I have always found, 

 in pulling them asunder, that the greatest resistance is offered at this part of their 

 articulation. The median furrow leads down to the large oval opening [e) of the radial 

 canal, which partly interrupts the great transverse ridge. Below tliis ridge is a fossa 

 (/) extending across the whole breadth of this surface, but especially deep in its median 

 portion, for the lodgment of the elastic ligament which antagonizes by its extensile 

 action the action of the flexor muscles. — ^The general disposition of the five pairs of 

 flexor muscles passing between the First and the Second Radials, is shown in Plate 

 XXXIV. fig. 2, m\ m\ 



34. On removing the dorsal surface of the Pentagonal Base by the careful application 

 of an acid (Plate XXXIII. fig. 1), or by making a section parallel to its dorsal surface, 

 and in the plane of the openings which we have noticed in the internal, external, and 

 lateral surfaces of each First Eadial (Plate XXXIV. fig. 1, A, b), we find that these all 

 belong to one system of Canals extending radially from the central space, and also 

 forming an annulus around it. The two apertures on the internal face of each First 

 Eadial (§33) lead to two canals which converge towards each other, and which very 

 quickly meet, so as to form a single canal («, a) which passes directly towards the 

 external margin ; whilst each of the converging canals gives off a large lateral branch 

 {b, b), which meets a corresponding branch in the adjacent radial ; and thus the five 

 great Radial Canals are intimately connected at their origin by an arrangement that 

 reminds the Anatomist of the " Circle of Willis" at the base of the Brain. 



35. Bosette. — The peculiarly-shaped circular plate which occupies the central vacuity 

 in the Pentagonal Base (Plate XXXIII. figs. 2, 9), and which is shown detached in 

 figs. 10, 11, may be described as consisting of a disk perforated in the centre, with ten 

 rays proceeding from it, five of these rays («, a) being triangular in form and nearly 

 flat ; whilst each of the other five {b, b) that alternate with these has parallel margins 

 inflected on its ventral aspect in such a mannef as to form a groove, whilst the ray curves 

 to its dorsal aspect in such a manner as to bring this groove to the periphery of the 

 rosette, and then terminates abruptly as if truncated. Around the central perforation 

 we sometimes find on the ventral surface an irregular raised collar, obviously con-e- 

 sponding to the central passage of the annulus of the pentagonal base ; but more com- 

 monly this is replaced by a number of vertical processes irregularly disposed (fig. 11). 

 Its diameter, in a full-grown specimen, is about "045 inch. When we look at this 

 "rosette" in situ (Plate XXXIII. fig. 9), we find that the five triangular rays are 

 directed to the sutures between the five Radials (a, a), their apices joining the con- 

 tiguous pairs of these, just between their two adjacent apertures leading to the radial 



