2i6 ZOOTOMY. 



ventral walls of the body between the posterior edge of the 

 sternum and the pubes. 



155. The rectus abdominis, a narrow paired muscle formed of 

 longitudinal fibres, forming with its fellow the median portion of the 

 ventral abdominal wall : it extends from the posterior border of the 

 sternum to the pubis ; its posterior portion is tendinous. 



156. The obliquus externus, a broad sheet of muscle forming the 

 lateral portion of the abdominal wall, and extending, in front, over the 

 posterior ribs : the muscle proper, consisting of fibres directed down- 

 wards and backwards, only extends about half way between the 

 posterior border of the sternum and the pubis, but is continued back- 

 wards to the latter bone by a strong sheet of connective tissue or 

 aponeurosis. 



157. The obliquus intemus, situated beneath the external oblique, 

 which must be removed to show it : it is almost co-extensive with the 

 external oblique, and like it, its posterior portion is aponeurotic : its 

 fibres are nearly longitudinal in direction. 



1 58. The transversus abdominis, lying beneath the internal oblique 

 and formed of transversely directed fibres, which are seen through the 

 posterior or aponeurotic portions of the obliqui. 



XXI. Make a median longitudinal incision through 

 the abdominal walls from the posterior edge of 

 the sternum to the vent : from the anterior end of 

 this cut make transverse incisions along the 

 posterior border of the sternum, taking care not 

 to cut too deeply and so injure the air-sacs 

 (§ 159) : carefully reflect the flaps of muscle thus 

 separated, and note : 



159. The (abdominal) posterior air-sacs, one on 

 either side of the body-cavity or coelome now exposed : 

 they are transparent thin-walled sacs, unnaturally distended 

 with air, and largely concealed at present by the great 

 omentum (§ 161) and the viscera (§ 162). 



160. The posterior-intermediate air-sacs, lying one 

 on each side immediately in front of the corresponding 



