SKULL — GENERAL 



57 



anterior part is probably presternal. Manubrium, mesosternum 

 and xiphisternum are usually differentiated, the exceptions being 

 noted below. The xiphisternum is never reached by the ribs; it 

 takes various shapes and is ossified to varying extents. As a rule 

 the sternebrae remain separate (fig. 64) with fibrous or even synovial 

 joints between them, or they may fuse, the extreme being a single 

 mesosternum as in most Primates. The 

 xiphisternum (lacking in Ornithorhynchiis, 

 Cetacea and sloths) is rarely forked (some 

 Edentates) but may have fenestrse. 



Among Edentates the sternum presents several 

 peculiarities. In Manids, which have a greatly 

 elongated tongvie, the xiphisternum is extremely 

 long, terminating, in the Asiatic species in a 

 broad, spade-shaped plate; in the African species 

 it forms two long and slender bars which reach to 

 the pelvic region where the two are united. In 

 both groups, and in the Myrmecophagidas where 

 the sternum is long, some tongue muscles are 

 attached to these extensions. Tamandua has 

 strong ventral processes developed from the 

 sternebrae, and the ribs articulate with these as 

 well as intersternebrally with the sternebrae. 



In Rodents the sternum is often prolonged as 

 a pair of horns in front of the preclavia which are 

 spoken of as episternal, although preformed in 

 cartilage. In whales, where the fore limbs have 



largely lost locomotor functions, the sternum is greatly reduced. In Odontocoetes 

 there are three separate sternal bones, usually called sternebrae, although 

 usually reached by seven pairs of ribs. Mystacocetes have but a single 

 bone, the manubrium, to which but a single pair of ribs are attached, and 

 which is sometimes continued back by a 'xiphisternal' process. The Sirenian 

 sternum consists of ossified manubrium and xiphisternum, while the mesos- 

 ternum, connected with three pairs of ribs, persists as cartilage. 



Fig. 64. — Pectoral girdle and 

 sternum of mouse, near birth 

 (W. K. Parker, '68). o, acro- 

 mion; cl, clavicle; g, glenoid 

 fossa; p, presternum; pc, pre- 

 clavia; .s, spine of sc, scapula; 

 ss, suprascapula; st, sternebrae; 

 X, xiphisternum. 



SKULL 



The skull is a distinctly vertebrate structure, nothing like it 

 occurring in any Invertebrate, while Amphioxus alone of the lower 

 Chordates has anything even remotely resembling it. Amphioxus 

 has a number of skeletal bars supporting the cirri around the mouth, 

 their bases forming a ring around the oral opening; and there are 



