STERNUM AND SKULL. 



303 



cartilage-bones, membrane-bones are largely developed ; and 

 the two groups of bones, though primitively distinct, become 

 intimately and immovably united to form a complex structure, 

 in which the several morphological components, and even 

 the boundaries of individual bones, are difficult to determine, 

 and of which the essential composition can only be ascer- 

 tained by a study of the embryonic stages. 



In the following description the membrane-bones are 

 described in connection with the cartilage-bones with which 

 they are most closely associated, and their names are printed 

 in italics. Though it is convenient to deal with the membrane- 



EM. B.0. B.S. PT.A. RL. 



M.N. 



Fig. 61. Lepus cuniculus. The skull from the right side. The middle 

 portion of the zygomatic arch and the right half of the mandible 

 have been removed, (a. m. m.) 



A, external pterygoid process of ali-sphenoid. AS, ali-sphenoid. B, internal 

 orbital foramen. BO, basi-occipital. BS, basi-sphenoid. c, occipital con- 

 dyle. D, mandibular symphysis. EO, ex-occipital. F, frontal. FA, foramen 

 lacerum anterius. FM, foramen ovale, confluent with the foramen lacerum 

 medium. G, orbital groove, for ophthalmic division of trigeminal nerve. I, 

 anterior upper incisor. IF, infra-orbital foramen. IP, interparietal. J, lower 

 incisor. i_,lacrymalbone. LF, lacrymal foramen. M, maxilla. MN, mandible. 

 N, nasal bone. OF, optic foramen. OS, orbito-sphenoid. p, parietal. PE, par- 

 occipital process of ex-occiptal. PL, palatine bone. PM, pre-maxilla. PO, 

 periotic. PT, pterygoid, s, squamosal. SF, stylo-mastoid foramen. SO, 

 supra-occipital. T, tympanic bones. ZM, zygomatic process of maxilla, cut 

 short. ZS, zygomatic process of squamosal, cut short. 



