This Ribs. 



73 



The sacrum exhibits many features resulting from its formation 

 through the fusion of originally distinct vertebrae. On the ventral, or 

 pelvic surface (facies pelvina), the lines of junction maybe traced either 

 between the bodies, or between the transverse processes. Four pairs of 

 apertures on this surface, the anterior sacral foramina (foramina sacralia 

 anteriora), lead into the intervertebral foramina, and give passage to 

 the sacral spinal nerves. On the dorsal surface (facies dorsalis) a pair 

 of posterior sacral foramina lie in the line of junction of the first and 

 second vertebrae. The spinous processes are evident in all four 

 vertebrae. The combined articular and mammillary processes are 

 conspicuous only in the first two, but are represented in the succeeding 

 two by low, roughened tubercles. In the middle line dorsally the 

 vertebral arches are separated by con- 

 spicuous apertures, the median sacral fora-' 

 mina. 



The caudal vertebrae (vertebrae caudales) 

 are sixteen in number. They are segments 

 of small size, increasing slightly to the third, 

 and then gradually decreasing to the end of 

 the column. The arches are complete in the 

 first seven. The transverse processes are 

 vestigial in all except the third. At the end 

 of the column the segments are reduced 

 to slender cylinders of bone representing the 

 vertebral bodies. 



THE RIBS. 



The ribs (costae) are twelve in number 

 on either side. Each is composed of a 

 dorsal portion, the costal bone (os costale), 

 or bone-rib, and a ventral portion, the costal 

 cartilage (cartilago costalis) (Fig. 29). From 

 their attachment on the vertebral column the 

 bone-ribs are directed outward, downward, 

 and backward. The costal cartilages are 

 directed for the most part inward, down- 

 ward, and forward. The first costal cartilage 

 forms a pronounced angle with the corres- 

 ponding bone-rib. In the succeeding ribs 

 the angle is gradually replaced by a broad 

 curve. 



Ribs are classified as true ribs (costae 

 verae), and false ribs (costae spuriae). The 

 former — comprising the anterior seven — - 



are those directly attached to the sternum. The latter — comprising the 

 posterior five — are either indirectly attached, or unattached. The 

 unattached ribs are designated as floating ribs. 



Generally speaking, the bone-ribs are cylindrical; but the anterior 



Fig. 29. The sternum and first 

 rib, ventral view: 1-7, the true 

 ribs; 8, first false rib; c.c, head 

 of rib; cl.c., neck of rib; cr., 

 costal cartilage; cr.c, body of 

 rib; c.s., body of sternum; m.s., 

 manubrium sterni; o.c, bone-rib; 

 p.x., xiphoid process; i.e., costal 

 tubercle. 



