DISSECTION OF THE DOG 11 



humerus. The whole of the muscle cannot be conveniently examined at the 

 present stage of the dissection. 



M. tbansveesus costabum. — This is a small, thin, irregularly triangular 

 muscle placed over the union of the bony and cartilaginous segments of the 

 first two or three ribs. Its origin is from the first rib, and its insertion, by 

 means of a thin and not very definitely bounded aponeurosis, is into the sternum 

 from the third to the fifth or sixth costo-sternal joint. 



Dissection. — Until such time as the inguinal canal has been examined the 

 abdominal wall should be dissected on one side only. Reflect the 

 external oblique abdominal muscle after making two incisions. The 

 first incision should be longitudinal and through the aponeurotic tendon 

 of the muscle a short distance from the fleshy margin. The second 

 incision must be transverse and through the fleshy part of the muscle 

 about midway between the thigh and the last rib. Now turn the aponeu- 

 rosis as far towards the middle line as possible. Note that, forming the 

 medial boundary of the superficial opening of the inguinal canal, there 

 is a stout band connected with the origin of the pectineus muscle. This 

 is formed by a blending of the aponeuroses of the two oblique and the 

 transverse abdominal muscles. Immediately cranial to this the external 

 oblique aponeurosis can be reflected to the middle line (linea alba), but 

 as the xiphoid region is approached the reflection becomes less and less 

 complete owing to a fusion with the internal oblique aponeurosis. 



M. OBLiQUXis intebnxjs abdominis.— Many of the fibres of the internal 

 oblique muscle of the abdomen run in a cranio- ventral direction, but towards 

 the region of the groin they become more and more transverse. The margin 

 of the fleshy part of the muscle forms a sinuous curve, the most caudal part of 

 which is superficial to the rectus muscle. The muscle has a dorsal, attachment 

 to the lumbo-dorsal fascia, the inguinal ligament, and the last two ribs. Ven- 

 trally it is continued as an aponeurosis in which two layers can be demonstrated. 



The superficial layer assists the aponeurosis of the external oblique muscle 

 in the formation of the superficial sheath of the rectus muscle, and thus reaches 

 the linea alba. The fibres of the two aponeuroses become interwoven medial 

 to an oblique fine stretching from the middle line of the body near the pubis 

 to about the middle of the breadth of the rectus muscle at the costal margin. 



The deep layer of the aponeurosis is incomplete and does not exist caudal 

 to the umbilicus. As will be seen later, it blends with the aponeurosis of the 

 transverse muscle of the abdomen to form the deep sheath of the rectus muscle. 



Dissection. — Make a longitudinal incision through the superficial sheath 

 of the rectus muscle along a line a few millimetres within the lateral 

 edge of the muscle. The incision should be made with care as the sheath 

 is not very thick. Turn the sheath towards the middle line, and in 



