DEVELOPMENT OF ANTEDON (COMATTJLA, LAMK.) EOSACEUS, 719 



Between the canal and the dorsal margin are shown the large elastic ligaments, l, i- 

 — In Plate XLIII. fig. 6 is shown a similar section of a decalcified Arm, passing 

 vertically along the median plane through the axial cord (a, a); so as to bring into view 

 the muscles (m, m) lodged in the spaces intervening between the vertical lamellfe (s\ s') 

 of the successive segments, the interarticular ligaments (l\ I') above the axial canal, and 

 the great elastic ligaments (P, l") below it, lodged in the deep fossae of the bodies of the 

 segments (s, s). — In fig. 7 is shown a longitudinal section of a decalcified Arm passing 

 horizontally through the axial cord (a, a) ; which exhibits the oblique disposition of the 

 elastic ligaments [P, P), occasioned by the alternating obliquity of the segments ; and also 

 shows the interposition of the sarcodic radiations at the syzygies (sg, sg). 



43. When the articular surfaces of the Brachial segments arc brought into view by 

 separating these segments from each other, we find them for the most part characterized 

 by the same features as those which are presented by the opposed surfaces of the First 

 and Second Radials (§§ 33, 37). With certain exceptions to be presently noticed, each 

 is divided by a transverse ridge (which usually crosses it more or less obliquely) embracing 

 the opening of the axial canal; and whilst the single deep fossa below this ridge 

 gives lodgment to the elastic ligament by which the arm is extended, the pair of shal- 

 lower fossae above it serve for the attachment of the interarticular ligaments ; whilst 

 above these, again, are the pair of deep fossae, formed by the recession of the vertically 

 projecting lamellae, in which the flexor muscles are lodged. From the peculiar shape of 

 the brachial segments (§ 41), the articular surfaces generally incline to one side or the 

 other, instead of looking directly forwards or backwards along the axis of the arm. 



44. It obviously results from the general conformation of the segments, the peculiar 

 disposition of their articular surfaces, and the arrangement of the muscles and ligaments, 

 that provision is made for very free flexion of the Arm towards the ventral aspect by the 

 contraction of its muscles ; and this flexion can take place to such an extent that the arm 

 may in a moment coil itself into a spiral resembling that of a watch-spring. When the 

 muscular tension is relaxed, the elasticity of the ligament on the dorsal side of each 

 segment straightens the arm, and may even flex it in some degree in the opposite 

 direction ; such dorsal flexion, however, never takes place to any considerable extent. 

 The arms also possess a slight power of lateral flexion, in virtue, as it would seem, of an 

 inequality of action in the two flexor muscles of each articulation ; but this flexion is 

 very limited in amount. 



45. The Axial canal by which each segment is perforated, carries on through the whole 

 length of the Arms the axial canal which divaricates in the Third Radial (§ 38) ; and 

 it also gives ofl" branches which in like manner enter the bases of the Pinnules, and 

 proceed through their successive segments to their extremities. This canal is occupied 

 by a solid cord of sarcodic substance, which I shall hereafter show to be really, like the 

 similar cords that fill the canals of the dorsal cirrhi (§ 29), a branch of the original 

 Crinoidal axis. 



46. The peculiarities presented by individual segments of the Arms are most conspi- 

 MDCCCLXVI. 5 F 



