138 Anatomy of Skeleton 



the second month chondrification commences in this, the mesenchyme comes into 

 relation with the vertebral region in the sixth week, and the cartilage is in position in 

 the seventh week. There are three centres of chondrification corresponding with the 

 three primary parts of the bone, and these fuse and form a shallow acetabulum in the 

 seventh week. By the end of the second month the two innominate plates meet at 

 the symphysis, and the pubic chondrification extends towards this later. 



Ossification commences now, that is at the end of the eighth week, as the first of 

 the primary centres, that for the Ilium. The centre for the Ischium is found just before 

 the fourth month, and that for the pubis about a month later. These primary centres 

 appear in their regions near the acetabulum. 



At birth each of these main centres has formed a small piece of the corresponding 

 acetabular wall, the rest of the hollow being cartilaginous, as are also the iliac crest, the 

 front margin of the great sciatic notch, the region of the ilio-pectineal eminence, the 

 region of the symphysis, and the ischio-pubic ramus. 



Ossification extends slowly in this cartilage, so that about the tenth or eleventh 

 year the ischial and pubic bones have met in the lower ramus, and just after this 

 acetabular centres appear in the triradiate cartilage, which still separates the three 

 primary bones as they have slowly extended on to the floor of the acetabulum. 



The acetabular centres appear to be very variable in their number : they may join 

 the neighbouring bones, or may fuse to form a single separate ossicle, the os acetabuli, 

 which ultimately fuses with the pubic mass. In this way the acetabular region is 

 consolidated by the age of puberty, fusion occurring first on the pelvic aspect ; but 

 the solid bone is still edged by cartilage on the crest, along the front and back borders 

 in places, in the region of the symphysis, and along the back part of the margin of the 

 lower ramus. 



In this cartilaginous border secondary centres appear as soon as the bone is 

 consolidated, at or just after puberty. They are : 



(1) For the anterior superior spine and front part of crest ; 



(2) For posterior superior spine and back part of crest ; 



(3) For anterior inferior spine ; 



(4) For spine of Ischium ; 



(5) For marginal surface of ischial tuberosity (hypoischium) ; 



(6) Somewhat later, for angle of symphysis ; and 



(7) For spine, apparently not constant. 



All these secondary centres are usually fused with the main mass by the age of 

 twenty or twenty-one, the last two being a few years later in consolidating. 



FEMUR. 



A long bone forming the skeleton of the thigh, articulating with the innominate 

 bone above and with the tibia below, and carrying the patella in front of its lower 

 end : consisting of a shaft, which is directed downwards, inwards and slightly forwards, 

 and an upper and lower end (Fig. 114). 



The upper end includes head, neck, and two trochanfers. The head is connected 

 with the shaft by the elongated neck, which is directed upwards and inwards and some- 

 what forwards,* forming an angle of about 125 degrees with the shaft. The head is 

 about two-thirds of a sphere, and has on it a depression, thefoveafemoris, for the attach- 



* Variable in amount of rotation ; may be even directed slightly backwards. 



