6 OBSTETRICAL ANATOMY. 



it communicates in front. It is formed of bony and ligamentous walls, 

 and contains, sustains, and protects a portion of the genito-urinary ap- 

 paratus, as well as the terminal portion of the alimentary canal. It is 

 situated towards the end of the spine, and is supported by the posterior 

 extremities, with which it is connected by joints and muscles. For the 

 hind limbs, as well as for some of the powerful muscles of the trunk, the 

 pelvis constitutes a most important fulcrum or fixed point in various 

 movements. It is one of the two mechanical elements concerned in the 

 act of parturition, and its form varies more or less in different species ; 

 though its direction is always rectilinear in the domesticated animals, 

 and not incurvated as in woman. 



SECTION I. BONES OF THE PELVIS. 



The pelvis is composed of three principal bones — the two coxce or ossa 

 innominata, and the sacrum ; and to a certain extent of the coccygeal or tail 

 bones. At an early period of life these bones can be subdivided, but after 

 a certain time they become consolidated. Each coxal bone, for instance, 

 is at an early stage of intra- uterine existence composed of cartilage only j 

 subsequently three centres of ossification appear, and these extend until 

 at birth they have coalesced to form three bones which are united by car- 

 tilage. In addition to these centres, two complementary nuclei are pres- 

 ent, one of which constitutes what is termed the atiterior iliac crest (k s^ifie 

 and the ilio-pectineal lifie or ridge^ and the other the ischiaiic tuberosity. 

 After birth, the three chief portions of the coxae are completely ossified, 

 and meet in the acetabulum — where they are closely joined — and at the 

 pubic symphysis, where the coxa of one side meets its fellow of the other. 

 In youth, the different parts of each coxa are very thick, the spongy tissue 

 being abundant, and the compact tissue scanty; as the animal advances 

 in age, the former diminishes and the latter increases iff density and 

 thickness, the two layers closely approaching each other. 



I. Os Innominatum. 



The coxa or os innominatum is a pair bone, there being one on each 

 side ; it belongs to the trunk, through its concurrence in the formation of 

 the pelvis, and also to the posterior limb, of which it constitutes the first 

 ray or haunch. 



It is a flat bone, widely expanded at either extremity, rather constricted 

 in the middle portion, and curved or twisted in two different directions, its 

 anterior part forming an obtuse angle with the posterior ; so that while its 

 external surface is inferior in the hinder portion, the anterior looks out- 

 wards, or even upwards, in front. At its middle portion it offers a wide 

 and deep articular depression surrounded by a high rim — the cotyloid cav- 

 ity or acetabulum — in which the corresponding articular head of the femur 

 is lodged and moves. Above this cavity is a marked, roughened thin 

 ridge — the supra-cotyloid crest or ischiatic spine, into which the sacro- 

 sciatic ligament is fixed. 



Below the cotyloid cavity, and inclining inwards, is a large circular or 

 oval aperture, which is occupied by the obturator muscles, and is named 

 t\iQ foramen ovale or sub-pubic openifig or foramen. 



The two coxae are united interiorly on the median line, and posteriorly 

 by an articulation or solid suture — syjnphysis pubis or ischio-pubic symphysis, 

 and form by this junction a,kind of V-shaped figure, the widest portion of 

 which is in front, and renders the lateral diameter of the pelvis more ex- 

 tensive before than behind. Above, they articulate with the sacrum. 



