2 OBSTETRICAL ANATOMY. 



genito-urinary apparatus, as well as the terminal portion of the ali- 

 mentary canal. It is situated towards the end of the spine, and is sup- 

 ported 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 ; subsequently three centres of ossification appear, and 

 these extend until at birth they have coalesced to form three bones, 

 which are united by cartilage. In addition to these centres, two com- 

 plementary nuclei are present, one of which constitutes what is termed 

 the anterior iliac crest or spine and the ilio-pectineal line or ridge, and 

 the other the ischiatic 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 in density and thickness, the two 

 layers closely approaching each other. 



1. 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 con- 

 stricted 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 poi'tion, the 

 anterior looks outwards, or even upwards, in front. At its middle 

 portion it offers a wide and deep articular depression surrounded by a 

 high rim — the cotyloid cavity 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 

 S2)ine, 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 the fora7nen ovale or sub-pubic opening or foramen. 



The two coxae are united inferiorly in the median line, and posteriorly 

 by an articulation or solid suture — the symphysis 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 extensive before than behind. Above, they articulate, 

 with the sacrum. 



