8 6 OSTEOLOGY. 



transverse abdominal and other muscles ; they are more developed 

 in some horses than in others, and occasionally project so far as 

 to form what are termed " ragged hips." 



The superior or internal border extends from the posterior 

 spine to the ischium. It is sharp and thin above, and becomes 

 thicker posteriorly, presenting above the acetabulum a roughened 

 ridge, the sciatic or superior ischiatic spine, the posterior por- 

 tion of which is formed by the ischium, and to which a large 

 part of the sacro-sciatic ligament is attached. 



ISCHIUM. 

 (PL. I. 14.) 



The ischium, intermediate in size between the ilium and the 

 pubis, is a flat, irregularly quadrilateral bone, which extends 

 from the acetabulum, and forms the posterior part of the pelvis ; 

 it presents a body, a shaft, and a ramus. 



The shaft, which forms part of the sciatic spine, also joins the 

 ilium in the acetabulum ; it is rounded, and below the spine has 

 a smooth surface over which the obturator internus and pyri- 

 formis tendons pass. It forms the external boundary of the 

 obturator foramen. 



The body, which is nearly horizontal, is flat and smooth, and 

 forms the posterior boundary of the obturator foramen. Inter- 

 nally, it is roughened, and joins its fellow of the opposite side, 

 forming the symphysis ischii ; posteriorly it is roughened for the 

 attachment of muscles, and possesses on its external part a 

 prominent projection, the tuberosity of the ischium, to which 

 several muscles are attached. The ridge or spine below, running 

 forwards from the tuberosity, is the inferior ischiatic spine, 

 while the posterior border of the bone is formed by a thick ridge 

 running from the tuberosity to the symphysis, forming with its 

 fellow the ischial arch, which, in the male, supports the crura of 

 the penis. The ramus, not well developed in the horse, is the 

 small branch which joins the pubis, and forms part of the inter- 

 nal boundary of the obturator foramen, the union between the 

 bones being usually marked by a transverse ridge. 



PUBIS. 



The pubis, which is the smallest division of the os innomin- 

 atum, is likewise irregular in shape, and forms with its fellow the 



