640 ANATOMY OF VERTEBRATES. 



relatively smaller in the male Echidna than in the Ornithorhyn- 

 chus. The gland is situated lower down, in the popliteal region, 

 between the insertions of the deep-seated fasciculi of the adductor 

 femoris and the origins of the gastrocnemius ; it is of subspherical 

 form, about the size of a pea, with a smooth exterior ; the excre- 

 tory duct, wide at the commencement, soon contracts into a fila- 

 mentary canal, which again enlarges to form a small reservoir for 

 the secretion just above the base of the spur. The duct is 

 accompanied and partly covered by the posterior tibial nerve. 



The spur is a round, curved, sharp-pointed cone, traversed by 

 a canal, continued from the reservoir, and opening on the convex 

 side of the spur a little way below the pointed extremity. 



The true nature and use of this apparatus has not yet been 

 determined. Its close analogy with the poison-apparatus in other 

 animals suggests a corresponding function ; but no well authenti- 

 cated case of symptoms of poisoning consequent upon a wound 

 inflicted by the spur has been recorded : it seems on the contrary 

 that the Ornithorhynchus possesses not the instinct of availing 

 itself, when attacked or annoyed, of a weapon so formidable as, 

 upon this theory, the spur must be. 1 



1 ccxxxrv". p. 236. 



