MUSCLES OF THE TBVNK. 239 



superposed layers. Eeckoning them from without inwards, these are 

 designated the great, or ext&i-nal oblique, the small, or internal oblique, the 

 great straight, and the transverse muscle. Covered outwardly by an expan- 

 sion of yellow fibrous tissue, the tunica abdominalis, and separated from 

 those of the opposite side by the white line (linea alba), a medium raphe ex- 

 tending from the sternum to the pubis, these muscles support the intestinal 

 mass, and by their relaxation or contraction adapt themselves to the varia- 

 tions in volume these viscera may experience. 



Preparation. — After placing the animal in the first position, a wide opening is to be 

 made in the pectoral cavity by the ablation of a certain number of ribs, which should be 

 divided inferiorly, above the costal attachments of the great oblique muscle. The heart 

 and lungs are removed; then an incision is made in the diaphragm, to allow the 

 digestive viscera contained in the abdominal cavity to be taken away. It is not abso- 

 lutely necessary, however, to empty that cavity, and if its contents be allowed to remain, 

 several punctures should be made in the large intestine to prevent the accumulation of 

 gas, and the too great distention of the abdominal parictes. 



These preliminary precautions having been adopted, then proceed in the following 

 manner : 



1. Eemove the skin from this region, and with it the p^nnioulus carnosus, in order to 

 study the external surface of the abdominal tunic. 2. The dissection of the great 

 oblique muscle is accomplished by removing the yellow fibrous envelope from the fleshy 

 portion of the muscle, together with the sterno-trochineus. The inguinal ring should 

 be exposed by the ablution of the dartos muscle, the sheath and penis, or the mammae. 

 3. On tlie opposite side, the small oblique is uncovered by excising the great oblique, 

 leaving, however, that portion of the aponeurosis which is mixed up with that of the first 

 muscle. 4. The latter having been studied, dissect the great straight muscle of- the 

 abdomen on the same side, in separating from the white line, by a longitudinal incision, 

 the aponeurosis common to the two oblique muscles, dividing this aponeurosis and the 

 fleshy portion of the internal oblique by another incision extending transversely from the 

 umbilicus to the middle of the lumbar region, and laying back one of the musculo- 

 aponeurotic sections on the thigh, the other on the ribs. 5. The transverse muscle is 

 dissected on the same side as the external oblique has been. To expose it, nothing more 

 is necessary than to make two incisions similar to the foregoing, but including the two 

 oblique and the straight muscle, throwing back the two portions as above. 6. Lastly, 

 open the entire abdominal cavity by cutting through the transverse muscle in the same 

 way; then study the muscular digitations of that muscle, the internal orifice of the 

 inguinal canal, and the leaf reflected from the aponeuiosis of the great oblique muscle. 



1. Abdominal Tunic. 



The vast expansion of yellow elastic fibrous tissue spread over the two 

 external oblique muscles of the abdomen is so named. 



Very thick towards the prepubic tendon of the abdominal muscles and 

 in the vicinity of the linea alba, this expansion gradually thins as it ap- 

 proaches the sternum, and disappears near the abdominal insertion of the 

 sterno-trochineus muscles. It also diminishes in thickness as it extends 

 from the linea alba ; and when it reaches the fleshy portion of the great 

 oblique muscle it becomes reduced to an extremely thin leaf, whose fasciculi 

 separate more and more from one another, until they completely disappear. 

 x\nteriorly, however, it is seen to be prolonged on each side to the posterior 

 digitations of the serratus magnus. Posteriorly, it furnishes some bundles 

 of fibres, which are detached from the surface of the common tendon, and ■ 

 are carried between the thighs to be lost on the internal crural muscles. 



The abdominal tunic is covered by the skin and the panniculus 

 carnosus, from which it is separated by an abundance of cellular tissue. In 

 the male, its external surface gives attachment to the suspensory ligaments 

 of the sheath, and to the dartos ; and in the female,-to the elastic capsule 

 which envelops each mammary gland. By its internal face, it closely 

 adheres to the aponeurosis of the great oblique muscle ; though it is easily 



