42 THE SKELETON OF THE HORSE 
tralia), which are larger than the dorsal series and diminish in size from first to 
last; they transmit the ventral divisions of the sacral nerves. 
The dorsal and ventral foramina communicate with the sacral canal and are 
together equivalent to the usual intervertebral foramina. 
The lateral borders are rough, thick in front, thin behind. 
The base (Basis ossis sacri) is directed forward, and is relatively very wide. 
It presents centrally the body of the first sacral segment, which is wide transversely, 
flattened dorso-ventrally, and has a rounded surface which articulates with the 
last lumbar vertebra through the medium of an intervertebral fibro-cartilage. 
The ventral margin projects shghtly, forming the promontory (Promontorium). On 
Fie. 19.—Sacrum or Horse; VentTRAL ViEw. 
I-V, Bodies of original five vertebre, marked off by transverse lines; /—4, ventral sacral foramina; 45, articular 
surface of body of first vertebra; 6,6, notches; 7, 7, surfaces of wings for articulation with transverse processes of last 
lumbar vertebra; 8, 8, wings; 9, auricular surface; 10, lateral border; 11, transverse process; 12, posterior orifice 
of sacral canal; 13, last spinous process. 
either side of the body there is a smooth notch, which, with one on the last lumbar 
vertebra, forms a large foramen for the passage of the ventral branch of the last 
lumbar nerve. Above the body is the entrance to the sacral canal, flanked by a 
pair of articular processes, which project upward and forward from the arch, and 
have concave surfaces medially for articulation with those of the last lumbar 
vertebra. Lateral to each of these is a smooth notch which is converted into a 
foramen by apposition with the last lumbar vertebra. The lateral parts of the base, 
the alae or wings (Ale sacrales), are strong prismatic masses with pointed ends. 
Each has in front a large, oval, slightly convex surface for articulation with the 
transverse process of the last lumbar vertebra. Posteriorly there is an elongated 
oval area which faces dorso-laterally; this is the auricular surface (Facies auricu- 
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