THE RIBS 25 



cut into in front and behind by the vertebral notches (Inoisura vertebraHs cranialis, 

 caudaUs). The notches of two adjacent vertebra' form intervertebral foramina 

 for the passage of the spinal nerves and vessels ; in some vertebrae, however, these 

 are complete foramina instead of notches. The laminae are plates which complete 

 the arch dorsally, uniting with each other medially at the root of the spinous 

 process. 



The body and the arch form a bony ring which incloses the vertebral foramen 

 (Foramen vertebrale); the series of vertebral rings, together with the ligaments 

 which unite them, inclose the vertebral canal (Canalis vertebralis), which con- 

 tains the spinal cord and its coverings and vessels. 



The articular processes, two anterior and two posterior (Processus articulares 

 craniales, caudales), project from the borders of the arch on either side. They 

 present joint surfaces adapted to those of adjacent vertebrae, and the remaining 

 surface is roughened for muscular and ligamentous attachment. 



The spinous process (Processus spinosus) is single, and projects dorsally from 

 the middle of the arch. It varies greatly in form, size, and direction in different 

 vertebrae. ' It furnishes attachment to muscles and ligaments. 



The transverse processes (Processus transversi) are two in number and project 

 laterally from the side of the arch, or from the junction of the arch and body. 

 In the thoracic region each has a facet for articulation with the tubercle of a rib 

 (Fovea costalis transversalis). They also give attachment to muscles and liga- 

 ments. 



Some vertebrae have also a ventral or haemal spine. 



Mammillary processes (Processus mammillares) are found in most animals on 

 the last thoracic and anterior lumbar vertelirae between the transverse and anterior 

 articular processes or on the latter. 



Accessory processes (Processus accessorii), when present, are situated between 

 the transverse and posterior articular processes. 



Development. — The vertebrae are developed by ossification in the cartilage 

 which surrounds the notochord and forms the sides of the neural canal. There are 

 three primary centers of ossification, one for the body and one for each side of the 

 arch. Secondary centers appear later for the summit of the spinous process 

 (except in the cervical region), the extremities of the transverse processes, and the 

 thin epiphyseal plates at the extremities of the body. 



Sometimes there are at first two centers for the bodj^ which soon fuse. The process of ossifi- 

 cation extends from the lateral centers to form not only the corresponding part of the arch, but 

 also the processes and a part of the body next to the root of the arch (Radix areas). In the horse 

 and ox the body and arch are usually fused at Ijirth, but the epiphyses do not fuse till growth is 

 complete. In the pig, sheep, and dog the body and arch are united at birth by cartilage (neuro- 

 central synchondrosis), but fuse in the first few months. 



THE RIBS 



The ribs (Costae) are elongated curved bones which form the skeleton of the 

 lateral thoracic walls. They are arranged serially in pairs Avhich usually corre- 

 spond in number to the thoracic vertebrae. Each articulates dorsally with the spine 

 and is continued ventrally by a costal cartilage. Those which articulate with the 

 sternum by means of their cartilages are termed sternal or "true" ribs (Costse 

 sternales s. verae); the remainder are asternal or "false" ribs (Costa? asternales s. 

 spurise). Ribs at the end of the series which have their ventral ends free in the 

 abdominal wall are named floating ribs (Costae fluctuantes). The intervals be- 

 tween the ribs are termed intercostal spaces (Spatia intercostalia). 



A typical rib^ consists of a shaft and two extremities. The shaft (Corpus costae) 



' The term is employed here, as is usual in descriptive anatomy, to designate only the bony 

 part of the rib (Os costale) ; morphologically it includes the cartilaginous part also. 



