no. 1636. ARM HOMOLOdins f\ ItKCVXT C/lfXOWFt— CLARK. 



115 



Fig. 1. — Articu- 

 lar PACE <>F A 



" S T R A I I! II T 



m u s c r i, a r" 



ARTICULATION. 



It is evident that there is a very radical difference between these 

 two types of articulation morphologically in their effect upon the 

 arm structure as well as in their composition. 



Muscular articulations fall naturally into two types, which, so Ear 

 as I have seen, are always perfectly distinct, and are 

 not interchangeable in position : a 



(a) Straight muscular articulations (fig. 1), which 

 have the transverse ridge separating the large dorsal 

 ligament fossa from the interarticular ligament fossa- 

 perpendicular to the dorso-ventral axis of the joint 

 face, and the two interarticular and muscular fossa 1 

 similar and equal in size. 6 This is the type of articu- 

 lation by which the radial articulates with the next 

 following joint, and it is never found beyond the distal 

 faces of the last axillary in any arm, and sometimes does not occur 

 even so far out as that. (See below under Metacrinus, Isocrinus, and 

 Comaster.) 



In an external dorsal view of an arm a straight muscular articula- 

 tion may be distinguished by having the two points of 

 contact of the two joints lateral and equidistant from 

 the median dorsal line (figs. 10 and 11). 



(b) Oblique muscular articulations (fig. 2), which 

 have the transverse ridge separating the large dorsal 

 ligament fossa from the interarticular ligament fossa' 

 strongly oblique (either to left or right) to the dorso- 

 ventral axis of the joint face, accompanied by a corre- 

 sponding distortion of the interarticular and muscular 

 fossa?. This type of articulation is first found at the 

 second articulation beyond the last straight muscular articulation, 

 and immediately succeeding the last synarthry (see below), and con- 

 tinues thence throughout the arm, except for the occasional inter- 

 polation of syzygies. 



Oblique muscular articulations are at once recog- 

 nizable in an external dorsal view of an arm (figs. L0 

 and 11) by having the two points of contact, represent- 

 ing the ends of the transverse ridge, one dorso-lateral 

 the other ventro-lateral ; when occurring on the distal 

 faces of axillaries (figs. 6 and 8) they may be distin- 

 guished from straight muscular articulations (figs. 3 

 and 4) by having the dorsal points of contact on either 

 side of the anterior angle of the dorsal surface of the joint, instead 

 of exactly at the anterior angle as is the case with straight muscular 

 articulations. 



Fig. 2. — Articu- 

 lar face of 

 an " oblique 

 m d scul ar " 

 articulation. 



Fig. :;. — An ax- 

 illary w 1 T II 

 "STEAIGH T 

 M D S C IL A R " 

 DISTAL PACES. 



a See footnote on p. 118. 



6 In the ease of straight muscular articulations on the distal faces of axil- 

 laries, the outer elements of the joint faces are somewhat cut away. 



