158 ZOOTOMY. 



The position of the hyoidean apparatus, clavicles, interclavicle, cora- 

 coid, and right half of the sternum, and of the pubes and ischia, are indi- 

 cated by dotted outlines. On the left side (right in the figure), the my- 

 lohyoid (m.hy), pectoral (/), and external oblique (o.ex) muscles are 

 removed, as well as the posterior part of the rectus ventralis (r.v) : by 

 the removal of the pectoral the left half of the sterum (st) is exposed. 



fa, biceps : cl, cloacal aperture : c.mn, cerato-mandibularis : d, del- 

 toid : e.i, external intercostal : m.hy, mylo-hyoid : mn, mandible : 

 o.ex, obliquus externus, superficial portion : o.ex' , deep portion of 

 the same : o.hy, omo-hyoid : o.i, obliquus internus : p, pectoralis : 

 per, peritoneum : r.i, rectus internus : r.l, rectus lateralis : r.v, rectus 

 ventralis : s.c.m, sterno-cleido-mastoideus : s.hy, sterno-hyoideus : $r, 

 serratus : st, sternum : st', its posterior prolongations. 



and joining the preceding portion at its insertion : and an internal and 

 posterior portion (rectus internus, r.i) of triangular form, lying 

 between the ventral portion and the peritoneum, attached behind to the 

 puboischiatic ligament and gradually thinning out in front. 



126. The obliquus externus (o.ex, o.ex'), a thin sheet 

 of muscle lying external to the rectus, and with its fibres 

 taking a direction downwards and backwards (the animal 

 being supposed to be in the normal attitude). 



It consists of two layers, a superficial one (o.ex) arising by slips from 

 the vertebral ribs, and with its fibres ending against the outer border of 

 the rectus lateralis, with which it becomes indistinguishable posteriorly ; 

 and a deep portion (o.ex') lying within (dorsal to) the pectoral, and, 

 similarly related to the rectus ventralis. 



127. The pectoralis (/), a large paired muscle arising 

 from the sternum and sternal ribs, which it largely covers : 

 the fibres of its anterior part pass outwards and backwards, 

 those of its posterior portion outwards and forwards, those of 

 its middle region directly outwards, and, converging, are 

 inserted into the ventral aspect of the humerus, thus forming 

 the chief depressor of the arm. 



128. The mylo-hyoid (m.hy\ a strong sheet of trans- 

 verse muscular fibres passing from ramus to ramus of the 

 lower jaw. 



