26 OUTLINES OF EQUINE ANATOMY. 



margin of foramen magnum, which it forms by means of a 

 concave, somewhat sharp margin. It presents three sur- 

 faces : the anterior internal surface is firm, and presents a 

 number of depressions for accommodation of the cerebellar 

 convolutions, from which it is separated by the membranes 

 of the brain. The posterior Surface is bound on either side 

 by the mastoid ridges, and meets the superior in forming a 

 prominent ridge with a backward inclination, crest or crista. 

 This surface is divided into two parts by a roughened line 

 extending from the centre of the crest towards foramen 

 magnum. To this the cordiform portion of ligamentum 

 nuchse is attached ; the concave roughened surfaces on 

 either side of it, scabrous pits, afford attachment to com- 

 plex! major and minor and recti capitis major and minor 

 postici. To the crest and mastoid ridges are attached 

 temporalis, obliquus capitis superior, levator humeri, 

 trachelo mastoideus, splenius, and (to the crest) some of 

 the auricular muscles. The superior surface of the bone 

 forms the posterior part of the forehead, and is roughened, 

 for from it commence the parietal ridges. Thus it affords 

 attachment slightly to temporalis on either side, and ante- 

 riorly presents an irregular margin dentated centrally 

 for union with os triquatrum, squamous laterally for union 

 with the parietal and more externally with the squamous 

 temporal bone. 



OS TRiaUATRUM is a small shield-shaped portion of 

 bone, with a smooth and flat superior surface and dentated 

 margins ; ant ero -laterally convex, meeting at a rounded 

 point anteriorly for union with the parietals. Posteriorly 

 concave, thus presenting two prominent posterior angles, 

 articulating here with os occipitis. Its iiuier surface 

 presents the ossific tentorium, a remarkable bony promi- 

 nence, with three concave surfaces, uniting to form three 

 sharp margins, one at each side, giving attachment to the 

 membranous tentorium, one anteriorly for attachment of 

 the posterior margin of falx cerebri. 



The PARIETAL BONES are flat and thin, presenting 

 two surfaces and five margins. The external surface is 

 convex, looking upwards and outwards. At its posterior 

 part it presents a few foramina for the passage of men- 

 ingeal arteries to the coverings of the brain. It is divided 

 into two parts by the parietal ridge, a slightly prominent 

 line of bone passing obliquely outwards from the anterior 



