94 GLOSSARY 



Or'bit (L. orbita, the track in which anything rolls). The 



socket of the eyeball. 

 OrTrito-na'sal nerve. The first division of the fifth cerebral 



nerve, which supplies the orbit and the nose. 

 Or'bito-sphe'noid(L. orbita, the orbit; Gr. sphen, a wedge; eidos, 



shape). An ossification of the skull in front of the exit 



of each optic nerve, known in human anatomy as the lesser 



wings of the sphenoid bone. 

 Or'gan (Gr. organon, an instrument). A structure in a plant 



or animal which is appointed to perform some definite 



duty, which duty is termed its function. 

 Organic (Gr. organon, an instrument). Possessed of organs. 



Applied to those substances which are or have been living. 

 Or'ganism (Gr. organon, an instrument). Any living thing, 



plant or animal. 

 Orga'non adamanti'nae (L. enamel organ). A term applied to 



the enamel germ of teeth. 

 O'rigin (L. origo, beginning, source). Applied to that end of a 



muscle which is fixed during contraction, the other end of 



the muscle being termed its insertion. 

 Ornithoderphia (Gr. arm's, omithos, a bird ; delphus, womb). 



- -The lowest division of the Mammalia, which therefore 



approaches nearest to the Sauropsida. 

 Ornithop'terous (Gr. arm's, ornithos, a bird; pteron, a wing). 



A genus of the Pterosauria. 

 Ornithoscelida (Gr. amis, ornithos, a bird ; skelis, skelidos, the 



haunch). A group of extinct Reptilia, intermediate in 



structure, (especially with regard to the pelvis,) between 



reptiles and birds. 

 Or'thidae (Gr. orthtos, straight, upright). A family of extinct 



Brachiopoda. 

 Orthog'nathous (Gr. orthos, upright ; gnathos, a jaw). A term 



applied to those skulls in which the craniofacial angle is 



small. 

 Orthop'tera (Gr. orthos, upright ; pteron, a wing). A division 



of the Insecta embracing the termites, cockroaches, grass- 

 hoppers, crickets, day-flies, dragon-flies, and earwigs. 

 Os articula're (L. articular bone). A bone in the mandible of 



some Vertebrata. 

 Os cal'cis (L. the heel-bone). Another name for the calcaneum 



or heel-bone. 



