THE THORAX THE OCCIPITAL BONE 



49 



ventral border is rounded, and is continued backward on the body of the bone. 

 The dorsal border is concave and has an articular cavity for the first pair of costal 

 cartilages. 



The posterior extremity is formed by the xiphoid cartilage (Processus xiphoideus) . 

 This is a thin plate, connected in front with the last bony segment by a relatively 

 thick, narrow neck, and expanding in nearly circular form behind and laterally. 

 Its dorsal surface is concave, and gives attachment to the diaphragm. The ventral 

 surface is convex and furnishes attachment to the transversus abdominis and the 

 linea alba. The free margin is very thin. 



Development. — At birth the sternum of the horse consists of seven bony seg- 

 ments termed sternebrse, which are united by in- 

 tersternebral cartilages. The last two sternebrae 

 fuse in the second month, but the others do not 

 usually unite completely even in old age. The 

 sternebrse consist of very vascular spongy bone 

 covered by a very thin layer of compact sub- 

 stance. The adult sternum thus consists to a />-''WL h'^ L . -First 



very considerable extent of persisting cartilage, ^<^S wf^'A. ^^"Itm 

 viz., the intersternebral cartilages, the ventral 

 keel, and the extremities; in old age these under- 

 go partial ossification. 



THE THORAX 



The bony thorax of the horse is remarkably 

 compressed laterally in its anterior part, but 

 widens greatly behind. The anterior aperture 

 (Apertura thoracis cranialis) is oval and very 

 narrow below; in a horse of medium size its 

 greatest width is about 4 inches (10 cm.), and its 

 height 7 to 8 inches (ca. 18-20 cm.). The ventral 

 wall or floor is about 16 inches (40 cm.) long, and 

 the dorsal wall or roof about 38 to 40 inches 

 (95-100 cm.) long. The height from the last seg- 

 ment of the sternum to the seventh or eighth 

 thoracic vertebra is about twice that of the anterior 

 aperture; this is due to the obliquity and diver- 

 gence of the roof and floor. The greatest width of 

 the posterior aperture is about 20 to 24 inches _ 



(50-60 cm.). The intercostal spaces (Spatia intercostalia) mcrease m width from 

 the first to the seventh or eighth, and then diminish. Their average width is 

 about IM to IH inches (3-3.5 cm.). 



First rib 



Cariniform 

 cartilage of 

 sternum 



Fig. 27. — Anterior Aperture of Tho- 

 rax OF Horse. (After Scliraaltz, Atlaa 

 d. Anat. d. Pferdes.) 



The Bones of the Skull 



(A) BONES OF the CRANroM 



The bones of the cranium (Ossa cranii) are the occipital, sphenoid ethmoid 

 interparietal, parietal, frontal, and temporal. The first three are single, the others 



paired. 



The Occipital Bone 



The occipital bone (Os occipitale) is situated at the posterior part of the cra- 

 nium of which it forms the posterior wall and part of the ventral wall or base.^ 

 1 The long axis of the skull is considered to be horizontal in these descriptions. 

 4 



