44 



THE SKELETON OF THE HORSE 



Tubercle 



and the intervertebral fibro-cartilage. The neck (Collum costae) is roughened 

 above and in front. The tubercle (Tuberculum costae) is placed above and behind 

 the junction of neck and shaft; it has a small surface (Facies articularis tuberculi 

 costae) for articulation with the transverse process of the corresponding thoracic 

 vertebra. 



The sternal extremity (Extremitas sternalis) is somewhat expanded, and is 

 continued by the costal cartilage. 



The first rib is easily distinguished. It is the shortest and least curved. At 

 the lower part of the anterior border there is a smooth impression where the brachial 

 vein curves around it; above this is usually the scalene tubercle. The costal 



groove is absent. The head is large and 

 has two facets of unequal extent which 

 meet at an acute angle in front; the smaller 

 one faces forward and articulates with the 

 last cervical vertebra; the larger one is 

 directed inward and articulates with the 

 first thoracic vertebra. The neck is thick 

 and very short. The tubercle is larger than 

 that of any other rib and has an extensive 

 articular surface which is convex in its 

 length. The sternal end is larger than that 

 of any other rib; it is thick and ver}^ wide, 

 and is turned a little forward. 



The last rib is the most slender and 

 regularly curved. It is usually but little 

 longer than the second. The facet on the 

 tubercle is confluent with that of the head. 

 (This feature, however, is usual on the 

 seventeenth also, and not rare on the six- 

 teenth.) 



The serial position of the other ribs may 

 be determined approximately by the follow- 

 ing considerations: The length increases 

 from the first to the tenth and eleventh 

 and then diminishes. The width increases 

 somewhat to the sixth and then diminishes. 

 The anterior border is thin and sharp from 

 the second to the eighth, and behind this 

 becomes thick and rounded. The external 

 groove is distinct on the fourth to the eighth 

 inclusive. The curvature increases in de- 

 gree rapidly from the first to the seventh, 

 remains about the same to the sixteenth, and 

 then decreases very noticeably. In regard 

 to dorso-ventral direction, the first rib inclines a little forward, the second is about 

 vertical, while behind this they slope backward in increasing degree, so that a 

 plane tangent to the ventral ends of the last pair cuts the third lumbar vertebra. 

 The head and tubercle diminish in size from first to last. Their relative positions 

 change in that tiie tubercle of the first rib lies almost directly outward from the 

 head, while further back it gradually comes to lie behind it. The neck is longest 

 on the longest ribs, and is absent on the last two or three. A costo-transverse 

 foramen is formed between the neck and the transverse process. 



Development. — The ribs ossify in cartilage from three centers, one each for the 

 shaft, head, and tubercle; the third center is absent in some of the posterior ribs. 



Fig. 22. — Lkit Skvknth Rib of Horse, Antkro- 

 EXTKRXAL ViEW. (After Schmaltz, Atlas 

 d. Anat. d. Pferdes.) 



