THE PELVIS. 213 
Architecture.—As a flat bone the os innominatum consists of spongy tissue between two 
compact external layers. These latter vary much in thickness, being exceptionally stout along 
the ilio-pectineal line and the floor of the iliac fossa immediately above it. The gluteal aspect 
of the ilium is also formed by a layer of considerable thickness. The spongy tissue is loose and 
cellular in the thick part of the ilum and in the body of the ischium ; absent where the floor of 
the iliac fossa is formed by the coalescence of the thin dense confining layers; fine grained and 
more compact in the tuberosity of the ischium, the iliac crest, and the floor of the acetabulum, 
in which latter situation it is striated by fibres which are directed radially to the surface of that 
hollow, these again being crossed at right angles by others which are arranged circumferentially. 
This spongy tissue forms a more compact layer over the surface of the upper and back portion of 
the acetabular articular area. The bottom of the floor of the acetabulum varies in thickness ; in 
most cases it is thin and in exceptional instances the bone is here deficient. The same condition 
has been met with in the iliac fossa, where absorption of the thin bony plate has taken place. 
Variations.—Some of the anomalies met with in the haunch bone are due to ossification of 
the ligaments connected with it ; in other cases they depend on errors of development. Failure of 
union between the pubic and ischial rami has also been recorded. Cases have occurred where the 
obturator groove has been bridged across by bone, and one case is noted of absence of the cotyloid 
notch on the acetabular margin. In rare cases the os acetabuli (see Ossification) remains as a 
separate bone. 
Ossification commences in the ilium about the ninth week of intra-uterine life ; 
about the fourth month a centre appears below the acetabulum for the ischium, the pubis 
being developed from a 
centre which appears in Appears about 
front of the acetabulum’ tt end of ead 
about the fifth or sixth | 
month. At birth the form 
of the ilium is well defined ; 
the body and part of the 
tuberosity of the ischium 
are ossified, as well as the 
horizontal ramus and part 
of the body of the pubis. 
All three parts enter into 
the formation of the sides 
of the acetabulum, and by 
the third year have con- 
verged to form the bottom 
of that hollow, — being 
separated from each other 
Appears about 15 
rears ; fuses 22-25 
Appears about 
; —15 years ; fuses 
22-25 years 
Appears about 
12 years 
Appears 
about 18 
years 
Appears 
about 18 
years 
/ 
i 5 s _ Appears about Appears about 
by a triradiate plece of 4th in. of fetal 5-6 m. foetal life 
cartilage, in which, about |" Appears about 157 \qy} 7 —~ Unite about 10 years 
a as i: years ; fuses 22- J 
the twelfth year, independ- 25 years 
ent ossific centres make At Birth. About 12 or 13 years. 
their appearance, which Fic. 157. —OssIFICATION OF THE INNOMINATE BONE. 
may or may not become 
fused with the adjacent bones. In the latter case they unite to form an independent 
ossicle, the os acetabuli, which subsequently fuses with and forms the acetabular part of the 
pubis. By the age of sixteen the ossification of the acetabulum is usually completed, 
whilst the rami of the ischium and pubis commonly unite about the tenth year. Secondary 
centres, six in number, make their appearance about the age of puberty, and are found in 
_ the following situations: one for the anterior inferior iliac spine, one for the iliac crest and 
the anterior and posterior superior iliac spines, a scale-like epiphysis over the tuberosity of 
the ischium, a separate epiphysis for the spine of the ischium, (?) a point for the spine 
and another for the angle of the pubis. Fusion between these and the primary centres 
is usually complete between the twenty-second and twenty-fifth years. 
‘ 
THE PELVIS. 
The pelvis is formed by the union of the innominate bones with each other in 
front, and with the sacrum behind. In man the dwarfed caudal vertebra 
(coceygeal) are curved forwards and so encroach upon the limits of the pelvic 
cavity inferiorly. The pelvis is divided into two parts by the ilio-pectineal lines, 
which curve forwards from the upper part of the lateral masses of the sacrum 
behind, to the roots of the spines of the pubes in front. The part above is called 
