Part 4

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=.

FIG. 3.--FOURTH THORACIC VERTEBRA, SIDE VIEW.

a, vertebral foramen; b, centrum; c, caudal, and d, cranial, costal demifacets; e, radix or pedicle; f, lamina; g, transverse process; h, cranial articular facet; i, caudal articular facet; j, caudal articular process; k, spinous process.]

At the caudal end of the centrum, at its dorsolateral angle, is a smooth area on each side continuous with the surface of the epiphysis and bounded dorsolaterally by a sharp ridge of bone (c). It is a =costal demifacet=. In corresponding positions at the cranial end of the centrum are two demifacets not limited by bony ridges (d). When the centra of two contiguous thoracic vertebræ are placed together in the natural position the cranial costal demifacets of one together with the caudal demifacets of the other form two =costal= facets (Fig. 4, e), one on each side, and each receives the head of a rib.

The dorsal two-thirds of the vertebral ring forms the =vertebral arch= which is continued dorsally into the long, bluntly pointed =spinous process= (Figs. 2 and 3, k) for attachment of muscles.

The vertebral arch (each half of which is sometimes called a =neurapophysis=) rises on each side from the cranial two-thirds of the dorsolateral angle of the centrum, as a thickened portion, the radix or pedicle (Figs. 2 and 3, e), which forms the ventral half of the lateral boundary of the vertebral canal. From the dorsal end of each radix a flat plate of bone, the =lamina= (f), extends caudomediad to join its fellow of the opposite side and form the vertebral arch. Owing to the fact that the radix rises from only the cranial two-thirds of the centrum there is left in the caudal border of the vertebral arch a notch bounded by the radix, the lamina, and the centrum. There is also a slight excavation of the cranial border of the radix. When the vertebræ are articulated in the natural position, these notches form the =intervertebral foramina= (Fig. 4, d), for the exit of the spinal nerves.

At the junction of radix and lamina the arch is produced craniolaterad into a short process, the =transverse process= (g), knobbed at the end. On the ventral face of its free end the transverse process bears a smooth facet, the transverse costal facet or tubercular facet (Fig. 4, c), for articulation with the tubercle of a rib.

On the dorsal face of each lamina at its cranial border is a smooth oval area, the =cranial articular facet= (superior articular facet of human anatomy) (Figs. 2 and 3, h). Its long axis is oblique and it looks dorsolaterad. The slight projections of the cranial edge of the laminæ on which the facets are situated are the inconspicuous =cranial articular processes= (prezygapophyses).

On the ventral surface of each lamina at the caudal border, near the middle line is a similar area, the =caudal articular= =facet= (inferior articular facet of human anatomy) (i); these occupy the ventral surfaces of two projections which form the =caudal= (inferior) =articular processes= (postzygapophyses) (j). These are separated by a median notch. When the vertebræ are in their natural position the caudal articular facets lie dorsad of the cranial facets and fit against them. They thus strengthen the joint between contiguous vertebræ, while permitting slight rotary motion.

a, spinous processes; b, cranial articular processes; c, transverse costal facets; d, intervertebral foramina; e, costal facets; f, accessory processes; g, mammillary processes; h, caudal articular processes.]

=Differential Characters of the Thoracic Vertebræ= (Fig. 4).--Following the thoracic vertebræ caudad there is to be seen a gradual increase in the size of the centra brought about by an increase in their craniocaudal and transverse measurements. The dorsoventral measurements remain nearly the same. The costal facets (Fig. 4, e) shift caudad so that on the eleventh, twelfth, and thirteenth thoracic vertebræ each lies entirely on the cranial end of its centrum, while the caudal end of the centrum immediately preceding is not marked by any part of it. In the eleventh thoracic vertebra each costal facet is usually still confluent with the smooth cranial end of the centrum. In the twelfth vertebra the facets are separated by smooth ridges from the cranial end of the vertebra, while in the thirteenth vertebra they are separated by rough ridges.

The spinous processes (a) of the first four are of about the same length. They then decrease in length to the twelfth, while the twelfth and thirteenth are slightly longer than the eleventh. The first ten slope more or less caudad, while the spinous process of the tenth (=anticlinal=) vertebra is vertical and those of the eleventh, twelfth, and thirteenth point craniad.

Each of the transverse processes of the seventh thoracic vertebra shows a tendency to divide into three tubercles; one of these is directed craniad, the mammillary process (or metapophysis), one caudad, the accessory process (or anapophysis), while the third (transverse process proper) looks ventrad and bears the transverse costal facet. This division becomes more prominent in the succeeding vertebræ, being most marked in the ninth and tenth. In the eleventh, twelfth, and thirteenth vertebræ the mammillary (g) and accessory (f) processes are very pronounced, while the transverse costal facet and that part of the transverse process which bears it have disappeared. The ribs of the eleventh, twelfth, and thirteenth vertebræ are thus attached to their respective centra by their heads alone.

The cranial articular processes (b) are prominent on the first two thoracic vertebræ; back of these they are very small as far as the eleventh, so that the articular facets seem to be borne merely upon the dorsal surface of the cranial edge of the laminæ. In the eleventh, twelfth, and thirteenth the cranial articular processes are large, bearing the articular facets on their medial surfaces, while the mammillary processes appear as tubercles on the lateral surfaces of the articular processes. The caudal articular processes (h) are prominent in the first thoracic, then smaller until the tenth is reached; in the tenth, eleventh, twelfth, and thirteenth they are large and their facets are borne laterally, so as to face the corresponding cranial facets. Thus from the tenth to the thirteenth thoracic vertebra rotary motion is very limited, owing to the interlocking of the articular processes.

=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.

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.

The =base= is slightly oblique and presents a smooth transversely oval articular facet (the cranial end of the centrum of the first sacral vertebra), for articulation with the centrum of the last lumbar vertebra. Dorsad of this is the sacral canal, more depressed than the vertebral arch craniad of it. It supports a spinous process (Fig. 6, a) which is directed dorsad. At the junction of its lamina and radix is seen the prominent cranial articular process (b) with sometimes slight indications of a mammillary process on its lateral surface. Laterad of the articular facet is seen the cranial face of the expanded “pseudo-transverse process” (c) of the first sacral vertebra. The ventral border of the base is concave ventrad, forming an arc of about 120 degrees. The =apex= shows the caudal end of the last sacral centrum. Dorsad of this are the vertebral arch with a very short spinous process (a′), and the caudal articular processes (d). Laterad of the centrum appears the laterally directed thin transverse process (e).

FIG. 7.--SACRUM, VENTRAL SURFACE.

Fig. 6.--1, 2, 3, the three sacral vertebræ. a, a′, spinous processes; b, cranial articular process of first sacral vertebra; c, expanded transverse process of first sacral vertebra; d, caudal articular processes of third sacral vertebra; e, transverse processes of third sacral vertebra; f, tubercles formed by fused articular processes of the vertebræ; g, dorsal (or posterior) sacral foramina.

Fig. 7.--1, 2, 3, the three sacral vertebræ. a, the transverse ridges formed by the union of the centra; b, cranial articular processes of first vertebra; c, transverse process of first vertebra; d, caudal articular processes of third vertebra; e, transverse processes of third sacral vertebra; f, f′, ventral (or anterior) sacral foramina; g, notch which helps to form third ventral sacral foramen.]

The ventral or pelvic surface (Fig. 7) is smooth, concave craniad, convex caudad, and crossed by two transverse ridges (a) along which are seen the ossified remains of the intervertebral fibro-cartilages. At the ends of the first ridge is a pair of nearly circular =ventral= (or anterior) =sacral foramina= (f) for the passage of sacral nerves. At the end of the second ridge is a pair of ventral sacral foramina (f′), smaller than the first pair and continued laterocaudad into shallow grooves for the ventral rami of the sacral nerves. That portion of the bone lying laterad of a line joining the medial borders of these two pairs of foramina is known as the =lateral mass= of the sacrum and is composed of the fused transverse processes of the sacral vertebræ. At the caudal margin of the ventral surface there is a notch between the lateral mass and the centrum (g). When the caudal vertebræ are articulated, this notch helps to form a foramen for the third sacral nerve.

The dorsal surface (Fig. 6) is narrower at its cranial end than is the ventral surface. Its cranial border bears laterally a pair of cranial articular processes (b) with their medially directed facets and between them it is concave, so that a large dorsal opening is left into the vertebral canal between the last lumbar vertebra and the sacrum. Caudad of the articular processes are two pairs of tubercles (f). These are the fused cranial and caudal articular processes of the sacral vertebræ. Caudad of them are the caudal articular processes of the last sacral vertebra (d). Craniolaterad of the middle and cranial tubercles are =dorsal= (posterior) =sacral foramina= (g) for the transmission of the dorsal rami of the sacral nerves. Three spinous processes (a) appear between these rows of tubercles. They decrease in height caudad. That part of the surface included between the spinous process and the tubercles is made up of the fused laminæ of the sacral vertebræ. That part between the tubercles and a line joining the lateral margins of the dorsal (posterior) sacral foramina is formed by the fused radices of the sacral vertebræ.