THE THORACIC VERTEBRA 



37 



the cervical vertcbne, and behind a cavity somewhat deeper than any other thoracic 

 vertebra. Two costal facets are found on either side, and a well-marked spine 

 ventrally. The arch is large and strong, and has large notches. The articular 

 processes are much larger than those of other thoracic vertebne, and resemble a 

 good deal those of the seventh cervical in form. The transverse processes are 

 short and thick, and each has on its ventral aspect a large concave facet for articu- 

 lation with the tubercle of the first rib. The spinous process is curved backward 

 and tapers to a point. Its length is usually about three or four inches (ca. 8 to 

 10 cm.). It may be mistaken at first glance for the last cervical, but is 

 promptly identified by the three costal facets on each side and the length of 

 the spine. 



The last thoracic vertebra is distinguished by the absence of the posterior pair 

 of costal facets, and the confluence of the anterior pair 

 with those on the transverse processes. 



The serial position of others may be determined at 

 least approximately by the following data: (1) The 

 bodies gradually diminish in length and width to the 

 middle of the region and then increase slightly. Their 

 costal facets become smaller and less concave from first 

 to last. The ventral crest is distinct on three or four 

 vertebrae at either end of the region. (2) The transverse 

 processes diminish in size and are placed lower down 



(i 



Spine ■ 



A n terior articu lar 

 processes 



Facet for 



tubercle 



of rib 



Facet for head 

 of rib 



nsvcrse process 



Tubercle 

 of rib 

 Neck 

 of rib 



Body of rib -^ 



Facets for 

 heads of ribs 



Articular cavity 

 of body 



Fig. 14. — Third Thoracic Vertebr.e axd Upper Part 

 OF Rib of Horse, Anterior View. (After 

 Schmaltz, Atlas d. Anat. d. Pferdes.) 



Fig. 15. — Lower Part of Ninth Thoracic Verte- 

 bra OF Horse, Posterior View. (After 

 Schmaltz, Atlas d. Anat. d. Pferde.s.) 



as they are traced backward. Their costal facets become smaller and lower 

 in position; on the last (and sometimes on its predecessor also) it fuses with 

 the costal facet of the body. The upper non-articular part of the process 

 gradually becomes more sharply defined, and in the last four or five separates to 

 form the mammillary process. (3) The spinous processes increase in length to 

 the third and fourth, antl then gradually diminish to the fifteenth, beyond which 

 they have about the same length. The backward inclination is most pronounced 

 in the second, the sixteenth is vertical, and the last two are directed a little forward. 

 The longest spines {i. e., those of the withers) are the thickest and have expanded 

 summits which remain more or less cartilaginous; the others are more plate-like, 

 and are surmounted by a thick lip. The second spine is more than twice as large 

 as the first. The summits of the fourth and fifth usually form the highest point 

 of the withers. 



Development. — There are six or seven centers, three for the body, two for 

 the arch, and one for the spinous process; some of the latter have an additional 

 center for the summit. 



