110 The Vertebrate Organ Systems 



The Pelvic Girdle and the Hindlimbs.— Like the pectoral girdle, 

 the pelvic girdle is formed of three pairs of bones, the ilium, the 

 ischium, and the pubis. In the adult mammal, these three bones are 

 immovably fused to form a solid bony mass known as the innominate. 

 At the anterior portion, the two parts fuse at the pubic symphysis. 



The three bones forming this girdle all meet at a large cup-shaped 

 depression, the acetabulum. It is into this depression that the large 

 femur of the upper hindlimb articulates. Above the acetabulum is the 

 ilium which terminates dorsally as the large crest. This crest forms 

 a place for the attachment of the heavy thigh muscles. Posterior to 

 the acetabulum is the heavy ischium, and anterior to the acetabulum 

 is the pubis. Between these two bones is the obturator foramen through 

 which the nerves and blood vessels of the legs pass. The pubis and 

 ischium fuse to form a symphysis below the obturator foramen. The 

 pubic bones from the two halves of the girdle fuse anteriorly at the 

 pubic symphysis. 



In quadrupeds, where the weight of the body is not borne by 

 the pelvic girdle, the spread of the various parts is not very great. 

 In upright mammals such as man, however, the curving nature of these 

 bones produces a basketlike arrangement which contains many of 

 the abdominal organs. The central opening of this girdle is some- 

 what larger in females than in males. This widening occurs at puberty, 

 and is correlated with the bearing of young by the female. At the 

 time of parturition, the cartilages at the pubic symphyses allow for ad- 

 ditional spreading of the girdle. 



The hindlimb is composed of a series of bones similar to those 

 of the forelimb. The most proximal of these is the femur, the head of 

 which articulates with the acetabulum of the pelvic girdle. Near the 

 head are the greater and lesser trochanters for muscle attachment. 

 Distally the femur has condyles and epicondyles similar to those of the 

 humerus. The bones of the lower leg consist of the large tibia and 

 the smaller, slender fibula. These articulate distally with a series of 

 small bones, the tarsals. One of these bones, the astragulus, articu- 

 lates with the tibia; another, the calcaneum, has a backward exten- 

 sion. These tarsals articulate distally with the long metatarsals which 

 in turn articulate with the phalanges. In human beings, the tarsals 

 form the ankle, with the calcaneum forming the heel, the metatarsals 

 form the foot, and the phalanges the toes. 



