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 sternebre, which are united by in- 
tersternebral cartilages. The last two sternebre 
fuse in the second month, but the others do not 
usually unite completely even in old age. The 
sternebre consist of very vascular spongy bone 
covered by a very thin layer of compact sub- 
stance. The adult sternum thus consists to a 
very considerable extent of persisting cartilage, 
viz., the intersternebral cartilages, the ventral 
keel, and the extremities; in old age these under- 
go partial ossification. 
First 
thoracic 
vertebra 
THE THORAX First rib 
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 em.), and its 
height 7 to 8 inches (ca. 18-20 cm.). The ventral 
wall or floor is about 16 inches (40 em.) long, and 
the dorsal wall or roof about 38 to 40 inches 
(95-100 em.) 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 yc. 27——Anrerron AperTcRE oF THOo- 
aperture; this is due to the obliquity and diver- Rax oF Horse. (After Schmaltz, Atlas 
gence of the roof and floor. The greatest width of Se eae 
the posterior aperture is about 20 to 24 inches 
(50-60 cm.). The intercostal spaces (Spatia intercostalia) increase in width from 
the first to the seventh or eighth, and then diminish. Their average width is 
about 114 to 114 inches (3-3.5 em.). 
Cariniform 
cartilage of 
sternum 
THE SKULL 
(A) BONES OF THE CRANIUM 
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.t 
oe long axis of the skull is considered to be horizontal in these descriptions. 
