The vertebral column, spinal column, or back-bone, consists of a varying
number of separate bones, the vertebræ. At its cranial end are seven
vertebræ (=cervical=, Fig. 1, _c_) which are without ribs and support
the head; caudad of these are thirteen rib-bearing vertebræ (=thoracic=,
Fig. 1, _m_); caudad of these are seven that are again without ribs
(=lumbar=, Fig. 1, _o_); these are followed by three vertebræ (=sacral=,
Fig. 1, _x_) which are united into a single bone, the =sacrum=, which
supports the pelvic arch. Following the sacral vertebræ are twenty-two
or twenty-three small ribless vertebræ which support the tail (=caudal=,
Fig. 1, _y_).
[Illustration: FIG. 1.--SKELETON OF CAT.
_a_, skull; _b_, hyoid; _c_, cervical vertebræ; _d_, clavicle; _e_,
scapula; _f_, sternum; _g_, humerus; _h_, radius; _i_, ulna; _j_,
carpus; _k_, metacarpus; _l_, phalanges; _m_, thoracic vertebræ; _n_,
ribs; _o_, lumbar vertebræ; _p_, innominate bones; _q_, femur; _r_,
patella; _s_, fibula; _t_, tibia; _u_, tarsus; _v_, metatarsus; _w_,
phalanges; _x_, sacrum; _y_, caudal vertebræ.]
=Thoracic Vertebræ.= =Vertebræ thoracales= (Fig. 4).--The =thoracic
vertebræ= are most typical, and the fourth one of these may therefore be
first described (Figs. 2 and 3). It forms an oval ring which has
numerous processes and surrounds an opening which is the vertebral
foramen (_a_). The ventral one-third of this ring is much thickened and
forms the =centrum= or body (=corpus=) (_b_) of the vertebra. The
centrum is a semicylinder, the plane face of which bounds the vertebral
canal, while the curved surface is concave longitudinally and is
directed ventrad. The dorsal plane surface of the centrum is marked by
a median longitudinal ridge on either side of which is an opening
(nutrient foramen) for a blood-vessel. The ends are nearly plane, the
caudal being slightly concave; they are harder and smoother than the
other surfaces. They may be easily separated in a young specimen as thin
plates of bone known as =epiphyses=.
[Illustration: FIG. 2.--FOURTH THORACIC VERTEBRA, CRANIAL END.