A =trochlea= is a pulley-like articular mass. A =glenoid cavity= (Cavitas glenoidalis) is a shallow articular depression, and a =cotyloid cavity= or =acetabulum= is a deeper one. The term =facet= is commonly applied to articular surfaces of small extent, especially when they are not strongly concave or convex. The terms =fossa=, =fovea=, =groove= or =sulcus=, and =impression= are applied to various forms of depressions. A =foramen= is a perforation for the transmission of vessels, nerves, etc. A =sinus= or =antrum= is an air-cavity. Other terms, such as =canal=, =fissure=, =notch=, etc., require no explanation.[3] VERTEBRAL COLUMN The =vertebral column= (Columna vertebralis) is the fundamental part of the skeleton. It consists of a chain of median, unpaired, irregular bones which extends from the skull to the end of the tail. In the adult certain vertebræ have become fused to form a single bony mass with which the pelvic girdle articulates. Vertebræ so fused are termed =fixed= or =“false” vertebræ= (Vertebræ immobiles), as distinguished from the =movable= or =“true” vertebræ= (Vertebræ mobiles). The column is subdivided for description into =five regions=, which are named according to the part of the body in which they are placed. Thus the vertebræ are designated as =cervical=, =thoracic= (or dorsal), =lumbar=, =sacral=, and =coccygeal= or =caudal= (Vertebræ cervicales, thoracales, lumbales, sacrales, coccygeæ). The number of vertebræ in a given species is fairly constant in each region except the last, so that the =vertebral formula= may be expressed (for the horse, for example) as follows: C_{7}T_{18}L_{6}S_{5}Cy_{15–21}. [Illustration: FIG. 4.—FIRST THORACIC VERTEBRÆ OF HORSE. To illustrate plan of structure of vertebræ. ] The vertebræ in a given region have special characters by which they may be distinguished from those of other regions, and individual vertebræ have characters which are more or less clearly recognizable. All typical vertebræ have a common plan of structure, which must first be understood. The parts of which a vertebra consists are the =body= or =centrum=, the =arch=, and the =processes=. The =body= (Corpus vertebræ) is the more or less cylindrical mass on which the other parts are constructed. The anterior and posterior extremities of the body are attached to the adjacent vertebræ by intervertebral fibro-cartilages, and are usually convex and concave respectively. The dorsal surface is flattened and enters into the formation of the vertebral canal, while the ventral aspect is rounded laterally, and is in relation to various muscles and viscera. In the thoracic region the body presents two pairs of demifacets (Foveæ costales) at the extremities for articulation with the heads of two pairs of ribs.