66 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Articular faces. — The apposed surfaces of two segments united by articulation, as 

 opposed to suture (see figs. 31-34, j). 71, and 36-40, p. 75, and pp. 113, 376); 

 (see Articulations). 



Articular facets — See Articular faces. 



Articulations. — The unions between adjacent ossicles when composed of ligament 

 bundles or of muscles, or of a combination of both (see Suture); articulations 

 are of two types, each ty])e being subdivided into two subtypes, as follows: 



A. Muscular articulations. — The apposed articular faces are marked 

 by an approximately hemispherical pit lodging the doreal ligament, 

 anterior (ventral) to which is a strong transverse ridge; sliglitly anterior 

 to the center of this ridge is the central canal through which passes the 

 axial cord of the dorsal nervous sj^stem; just anterior to the transverae 

 ridge lies a pair of intcrarticular hgament fossae, one on either side of the 

 central canal; these intcrarticular ligament fosssB are boimded anteriorly 

 by strong oblique ridges wliicli separate them from the pair of muscular 

 fossae (see figs. 31, 32, p. 71, 431, 432, p. 349 and pp. 114, 376). 



a. Straight muscular articulation (often known simply as Muscular 

 articulation). — A t^'pe of muscular articulation in which the transverse 

 ridge is perpendicular to the doreoventral axis of the joint face, and 

 the dorsoventral axis divides the joint face into two equivalent and 

 sunilar halves (see figs. 31, p. 71, 431, 432, p. 349, and pp. 114, 376^ 



b. Oblique muscular articulation. — A type of muscular articulation 

 in which the transverse ridge is strongly obUque in reference to the 

 doreoventral axis of the joint face (t\'pically making with it an angle 

 of 45°) and the intcrarticular and muscular fossae of the two sides 

 are more or less unequal (see figs. 6, p. 63, and 30, 32, p. 71 ). 



B. Nonmuscular articulations. — Articulations in which muscles are 

 absent, the imion being effected solely by hgaments (see figs. 33, 34, p, 71, 

 36-40, p. 75, and p. 113). 



a. Synarthrij. — A type of non-muscular articulation in which the 

 apposed articular faces show two hemispherical fossae for the recep- 

 tion of a pair of ligament bundles, separated by a strong ridge r unnin g 

 in the direction of tlie dorsoventral axis of the joint face, which is 

 pierced m the center by the central canal (see figs. 6, p. 63, 14, p. 65, 

 30, 33, p. 71). 



b. Sijzygy. — A tj-pe of nonmuscular articulation in wliich the 

 apposed surfaces are flat, and arc marked by fine low radiating ridges 

 (see figs. 2, p. 61, 6, p. 63, 14, p. 65, 34, p. 71, and 35, p. 73). 



(See also Cry ptosynarthry and Pseudosyzygy.) 



Asterise. — Same as Pentacrini. 



Autotomy.—{l) A process by which a comatulid inflicts self-mutilation, usually by 

 breaking off a part or all of an arm ; this usually occurs at either a syzj-gy or 

 at a synartliT}'. This process of autotomy in the crinoids has commonly been 

 supposed to be voluntary, but is in reahty the result of a state of panic which 

 causes a total relaxation of the muscles (see pp. 140-142). 



