MYOLOGY 43 



narrower than that of the left. The two shps of either 

 muscle join immediately craniad of the decussation and the 

 single muscle thus formed upon either side passes in con- 

 tact with the midventral line or linea alba to the xiphoid 

 cartilage, at which point it diverges slightly from the mid- 

 ventral line to its insertion upon the first costal cartilage, 

 and to a slight extent, the manubrium. In a total of four 

 specimens of Homodontomys in which this muscle was in- 

 vestigated the decussation of the right muscle was found 

 invariably to be superficial to the left, but it is not un- 

 likely that there is some individual variation in this re- 

 spect, as in the other subgenera. The tendency for the 

 divergence of the rectus from the midventral line in the 

 thoracic region was found to be greatest in Homodontomys 

 and least in Teonoma, with Neotoma intermediate in this 

 respect. 



In Neotoma decussation of the left muscle was found to 

 be superficial to the right in two specimens, and the reverse 

 in one. In all three the more superficial slip was only about 

 half as wide as the one beneath it. In Teonoma the decus- 

 sation of the left muscle was also found to be superficial 

 to the right in two specimens and the reverse in one. In a 

 single instance the more superficial slip was less than half 

 the width of the one beneath it, but both were of ap- 

 proximately^ equal width in the other two. 



The three muscles covering the side of the abdomen, 

 which at some points are separable only with extreme 

 difficulty, comprise the following. 



M. obliquus abdominis externus (figs. 6, 7, 9) has 

 origin by slips from the last nine ribs interdigitating wdtb 

 those of the serratus magnus as far as this extends, and 

 from the ectal surface of the lumbodorsal fascia. Inser- 

 tion is for the most part upon the sheath of the rectus 

 abdominis just mediad to the lateral margin of that muscle, 

 and upon Poupart's ligament. The latter extends from 



