=The Lumbar Vertebræ.= =Vertebræ lumbales= (Fig. 5).--The last thoracic vertebræ form the transition to the typical lumbar vertebræ. These are larger than the thoracic vertebræ. The centra are of the form of the centra of the thoracic vertebræ, and increase in length to the sixth, but the seventh is about the length of the first. They increase in breadth to the last. [Illustration: FIG. 5.--LUMBAR VERTEBRÆ. _a_, cranial articular processes; _b_, mammillary processes; _c_, caudal articular processes; _d_, accessory processes; _e_, transverse processes; _f_, spinous processes.] The cranial articular processes (Fig. 5, _a_) are prominent and directed craniodorsad; they have the facets on their medial surfaces, while their dorsolateral surfaces bear the mammillary processes (_b_) as prominent tubercles. The caudal articular processes (_c_) are likewise large; their facets look laterad. When the vertebræ are articulated they are received between the medially directed cranial processes. The accessory processes (_d_) are well developed on the first vertebra, diminish in size to the fifth or sixth, and are absent on the seventh and sometimes on the sixth. The transverse processes (more properly pseudo-transverse processes) (_e_) arise from the lateral surface of the centra; are flat and are directed ventrocraniolaterad. The first is small, and they increase in length and breadth from the first to the sixth, those of the last being slightly smaller than in the sixth. The free ends of the last four are curved craniad. The spinous processes (_f_) are flat and directed craniodorsad. They increase in length to the fifth and then decrease. The first five are knobbed at the end. In a dorsal view the spinous process and cranial articular processes of each vertebra are seen to interlock with the caudal articular processes and accessory processes of the preceding vertebra in such a way as to prevent rotary motion, and this arrangement may be traced craniad as far as the eleventh thoracic vertebra. =Sacral Vertebræ.= =Vertebræ sacrales= (Figs. 6 and 7).--The three sacral vertebræ are united in the adult into a single bone, the =os sacrum=, or =sacrum=. In a kitten the three vertebræ are separate, while in an animal almost mature the first two are united and the third is still separate. The sacrum lies between the last lumbar and the first caudal vertebræ and articulates laterally with the two innominate bones. It is pyramidal, with the base of the pyramid directed craniad, and is perforated by a depressed longitudinal canal, the =sacral canal=, which is a continuation of the vertebral canal, and by four large foramina dorsally and four ventrally. It may be described as having a cranial end or base and a caudal end or apex, a dorsal, a ventral, and two lateral surfaces.