116 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXV. 



Nonmusculav articulations fall also into two types; but, contrary to 

 what we found to be the case in muscular articulations, the second 

 type may partially or wholly replace the first 

 in a given arm, though the reverse is not true : 

 (a) Synarthries or bi fascial articulations 

 (fig. 5) : these are distinguished by having 

 the joint faces with a pair of large shallow 

 pits, separated by a ridge which traverses the 

 joint face along its dorso-ventral axis; this is 

 the type of articulation which is always found 

 on the distal end of a joint the proximal end of 

 which is united to the preceding by a straight 

 muscular articulation, and occurs nowhere else,' 

 any, or all, synarthries in an arm may be re- 



FlG. 4. DORSAL VIEW OF 



AN AXILLARY UNITED TO- 

 THE PRECEDING JOINT 

 BY " SYNARTHRY," AND 

 TO THE TWO SUCCEED- 

 ING BY " STRAIGHT MUS- 

 CULAR" ARTICULATIONS ; 

 THE TWO POST-AXILLARY 

 JOINTS ARE UNITED BY 



SYNARTHRY. 



Fig. 5. — Artic- 

 ular face i >F 



A " S Y N A R- 

 THRY." 



placed by syzygies. The most distal synarthry 

 in an arm is always immediately followed by an oblique muscular 

 articulation, as stated above. 



Synarthries are readily distinguishable in*i dorsal ex- 

 ternal view of an arm by having the points of contact 

 exactly in the median dorsal line (figs. 4, 10, and 11). 



(b) Syzygies (fig, 7) ; the joint faces are unmarked, 

 or are marked with striations radiating outward from 

 the central canal, the articulation being extremely close, 

 effected by numerous short ligament fibers which are not segregated 

 into bundles. Syzygie* may replace any or all syn- 

 arthries, and occur at intervals throughout the arm. 

 Syzygies are at once recognizable dorsally by the 

 extreme closeness of the articulation, which appears 

 as a very fine or dotted line. In drawings syzygies 

 are always represented by dotted lines (figs. 10 and 11). 

 From the above discussion it is evident that there 

 are two joints in each arm which, by their mode of 

 articulation with each other and their neighbors, are 

 sharply differentiated from all the other joints; I refer to the joints 

 on either side of the last synarthry; these joints 

 have articulating faces as follows: a straight mus- 

 cular articulation, binding the first to the preceding 

 joint, a synarthry, by which the joints are bound 

 together, and an oblique muscular articulation, 

 which binds the more distal of the two to the suc- 

 ceeding joint. Of course, as has been mentioned, the 

 synarthry may be replaced by a syzygy; but there is 

 no difficulty in distinguishing the pair even in that 

 case, for it is the only syzygial pair united to the preceding 



Fig. 6. — An axil- 

 l a r y W I T n 

 " OBLIQUE MUS- 

 CULAR " DISTAL 

 FACES. 



Fig. 7. — Articu- 

 lar face OF A 



" S T Z T G Y " 



(ADAPTED FROM 

 CHADWICK). 



