24 



THE SKELETOX 



cess and the neck of the rib enclose an arterial foramen, the costo-transverse. In the cervical 

 reirion this rib, or costal element, and the transverse process are fused together, but the conjoint 

 I)rocess thus formed is jnerced by the costo-transverse foramen. The compound nature of the 

 l)rocess is indicated by the fact that the anterior or costal processes in the lower cei-vical vertebrae 

 arise from additional centres and occasit)nally retain their independence as cervical ribs. These 

 l)rocesses in Sauropsida (birds and reptiles) are represented by free ribs. In the lumbar region, 

 the compound nature of the transverse process is further marked. The true transverse process is 

 sreatly suppressed, and its extremity is indicated by the accessory tubercle. Anterior to this in 

 the adult vertebra a group of holes represents the costo-transverse foramen, and the portion in 

 Iront of this is the costal element. Occasionally it will persist as an independent ossicle, the 



lumbar rib. t,. i • i /. 



In the sacral series the costal elements are peculiarly modified m the first three vertebrae to 

 form piers of bone for articulation with the ilium. The costo-transverse foramina are completely 

 obscured. In rare instances the first sacral vertebra will articulate with the ilium on one side, 

 but remain free on the other. Under such conditions the free process exactly resembles the 

 elongated transverse jjrocess of a lumbar vertebra. The first three sacral vertebrae which develop 

 a costal iirocess (rib) for articulation with the ilium are true sacral vertebrae. Those ankylosed 

 below these are pseudo-sacral. A glance at fig. 29 will show the homology of the various parts of 

 a vertebra from the cervical, thoracic, lumbar and sacral regions. 



The mammillary processes are vestiges of the greatly elongated articular processes of such 

 mammals as the dog, armadillo, &c. 



THE BONES OF THE SKULL 



The skeleton of the head is called the skull : it contains, in the adult, twenty- 

 nine separate bones. For descriptive purposes they are divided in two groups : 

 those of the skull proper, and the appendicular elements. 



I. The Skull 



r Occipital. 

 (a) Basilar Bones J Sphenoid. 

 I Temporals 



(/>) Roof Bones 



r Parietals. 

 -{ Frontal. 

 I Epipterics. 



(r) Nasal Region 



((/) Facial Bones 



Maxilla?. 



Palatines. 



Malars. 



[- Ethmoid. 



I Sphenoidal tur- 



binals. 

 ! Turbinals. 



Lachrymals. 



Vomer. 

 I Nasals. 



Mandible. 

 Malleus. 



II. Appendicular Elements 



Incus. Hyoid. 



Stapes. Styloid. 



Internal pterygoid. 



The epipterics are not always separate in the adult skull: the s^^/ok/ ankyloses 

 with the temporal, and the internal •pterygoid Avith the sphenoid. 



THE OCCIPITAL 



This bone forms tlie l)ack and a portion of the base of the skull. At birth it 

 consists of four distinct parts disposed around the foramen magnum (fig. 34). 

 These, in the adult, fuse together and form a single bone, which ankyloses with 

 the sphenoid. The four ])arts of the occipital are — the squamo-occipital, two 

 ex-occipitals, and a basi-occipital. The lines of union of these parts are easily 

 distinguished even in the oldest skull. 



