48 STRUCTURE OF THE SHEEP 



vertebrae, but Sisson recognizes only 4. With the exception of the 

 last vertebra, these are fused into one bone in the adult animal. 

 The number of coccygeal vertebrae present largely determines the 

 length of the sheep's tail, and that this number varies is apparent to 

 any one who has handled a large number of lambs before their tails 

 have been cut off. 



The Ribs. Ordinarily the sheep has 13 pairs of ribs, 8 pairs 

 being sternal or true ribs, 4 pairs asternal or false ribs, and one 

 pair floating ribs. True ribs are distinguished from false ribs by 

 the fact that the cartilaginous bars extending from them articulate 

 with the sternum or breast-bone, while those of the false ribs over- 

 lap and are attached to each other, forming what is known as the 



FIG. 18. Skeleton of sheep. (Courtesy of Win. R. Jenkins Co.) 



costal arch. The cartilages extending from floating ribs are unat- 

 tached. Sometimes there are 14 pairs of ribs, in which case the 

 fourteenth rib is also floating. 



The sternum, or breast-bone, is composed of 7 segments (some- 

 times only 6) and there are indentures in the sides for the reception 

 of the cartilages extending from the ribs. 



Effects of Variations in Skeletal Structure. No attempt will 

 be made to describe the bones of the skull and of the thoracic and 

 pelvic limbs because the chief object of discussing the skeleton in 

 this connection is to show how the external form may be influenced 

 by differences in those regions of the skeleton subject to variation in 

 number of parts. Undoubtedly many sheep which are relatively 

 longer than others have more than the average number of vertebras 



