904 MR. F. E. BEDDARD ON THE ANATOMY [DeC. 15, 



from the drawings (text-figs. 176 &, 177, pp. 899, 901) that, as in 

 Brevicejjs *, the boundaiy-line between the thigh ventrally and the 

 trunk ventrally is entirely occupied by these muscles, a separate 

 obliquus externus not being visible on this view of the animal. 

 Whereas in Rana, when the skin is reflected from the abdomen and 

 thigh, the obliquus externus as well as the rectus are seen to form 

 the boundary-line between limb and trunk. There is another 

 important difference which this muscle shows and in which it. re- 

 sembles Breviceps. The rectus abdominis overlaps a considerable 

 portion of the thigh, to the extent indeed of 6 mm. or so. Under 

 the free edge of the muscle latei'ally a seeker can be pushed. 

 There is, however, a plain distinction laterally between the rectus 

 abdominis and the obliquus muscle (for the moment I leave it 

 undecided whether it is to be regarded as externus or inteiiius), 

 which is not merely the lateral and dorsal extension of the rectus. 

 It will be noticed that the one inscriptio tendinea (see text-fig. 

 177) which exists behind the origin of the pectoralis abdominis, 

 and along the course therefore of the I'ectus abdominis, does not 

 reach the edge of the muscle which overlaps the thigh musculatvu'e 

 and towards which it tends. In this region then it is impossible 

 to discriminate between i-ectus and obliquus, on the assumption, 

 that is, that we have here reached the border-line of the two. 

 I am disposed, however, to think that this lateral extension of the 

 rectus is wholly rectus ; for a careful dissection shows that it ends 

 by being inserted upon the skin and its fibres are not continuous 

 with those of what is obviously the obliquus muscle described 

 above as originating from the dorsal aponeurosis. 



When the pectoralis abdominis is cut through and reflected 

 the anterior portion of the rectus abdominis is brought into Adew. 

 This lies at a much lower plane than the posterior region of the 

 muscle. For thei-e is a deep cavity between it and the covering 

 pectoralis abdominis. This cavity is not merely a lymph-space. 

 It contains an elongated body which I describe later in connection 

 with the thymus t. This cavity then is floored (examined in the 

 ordinary position of these muscles when dissected from the vential 

 surface) by a delicate layer of muscles (text-fig. 177, r.) which is by 

 far thinner than the rectus abdominis of which it is the forward 

 continuation from the anterior inscriptio tendinea. The muscular 

 fibres, however, do not extend over the whole of the cavity thus 

 exposed. Towa,rds the middle line the muscular fibres form an area 

 which is not only thicker in its muscular tissue than more laterally, 

 but definitely arises fi-om the inscriptio tendinea. Laterally there 

 is no such origin from the inscriptio tendinea where the rectus 

 abdominis and the pectoralis abdominis meet, and this sheet has 

 been desci-ibed as a part of the obliquus internus. 



The lateral portion of the rectus abdominis, under which, when 

 it covers the thigh, a probe can be passed, as already mentioned, 

 demands a more detailed consideration. It is to be noted, in the 



* P. Z. S. 1908, p. 16, text-fig. 3. f See p. 915. 



