Dissection of Common Rat. 183 



num, posteriorlv, the whole length of the abdomen. This is 

 the fibrous cord called Linea Alba; about midway in its 

 length is an enlarged white spot, which is the remains of 

 the Umbilicus. Situated on either side of, and attached to 

 the Linea Alba, on the inferior and middle part of the abdo- 

 men are the Recti [s. Sterno-pubiales) muscles, bounded su- 

 perficially by a tendinous aponeurosis. 



External to the Recti Muscles on either side is a white 

 line, marking both the outer edge of these muscles and the 

 union of the fleshy and aponeurotic portions of the pair of 

 muscles that together occupy the whole area of the abdomen, 

 viz.y the External Oblique (s. Costo-abdominalis) muscles. The 

 External Oblique may be traced upwards to its origin by 

 fleshy processes from the outer surface of several of the poste- 

 rior ribs ; and the alternation of these processes with similar 

 processes which form the origin of the Serratus Magnus* 

 muscle beneath the arm should be noted. 



From the front of the Sternum there arises the Pectoralis 

 Major muscle, triangular in shape, with the base at the Thorax^ 

 and the apex at the Arm, where it is inserted into the Humerus. 



The External Oblique tnuscle should be now raised^ by carry- 

 ing the scalpel through the digitations near the ribs, continu- 

 ing the incision along the side, and reflecting it towards the 

 middle of the body, in order to expose {a) the Eibs and the 

 Intercostal Muscles which lie just within its anterior fibres ; 

 and {b) the Internal Oblique muscle posteriorly. In raising 

 it care must be taken not to detach the Rectus muscle from 

 the Eibs above, and to avoid cutting through the tendon of 

 the Internal Oblique at the upper part, towards the sternum. 



The Internal Oblique muscle (s. Ilio-abdominalis ) rises 

 from the crest of the Ilium, and its fibres have a general 

 oblique direction forwards towards the middle line ; it is in- 



* The Serratus Magnus arises by pointed processes from the ribs, and is in- 

 serted into the Scapula. 



