32 



The origiiiiil ronditiuii oi' Meiiilir. iii(crii)., M. olilicj. iiil. and M. 

 Iraiisv. has least changed with Ateles paiiisciis. Witli tliis nioiikej 

 the M. transv. and the M. ohliq. int. still show their oi'iginal rela- 

 tion to the ]\I. iT'ctus; the Meinhr. iiilerni. lies in the cranial part 

 also lichind the i\I. rectus, [tasses then, however, into the perimysium 

 externum ol' tiiat muscle. This relation must be regarded as a con- 

 dition, in which tlie M. rectus is situated in the mass of the mem- 

 i)rane, in other words: there exists here a beginning piercing of the 

 membrane by the M. rectus. Willi the other Platyrrhini the piercing 

 process has gone further than with Ateles paniscus, and the mem- 

 brane lies then in the caudal part befoi'e the M. rectus. At the same 

 tinu' the piercing-process has with them extended over the M. obliq. 

 int.; the M. transversus, however, passes still entirely behind the 

 M. rectus. 



The monkeys of the old world have removed farthest from the 

 original con<lition of the structure of their sheath : with them the 

 piercing of the Menibr. inform, and of the M. obliq. int. is complete, 

 whilst the M. transv. in the caudal part also modifies its direction 

 with regard to the M. rectus. The structure of the sheath of the 

 .M. rectus of man forms the coimecting link between those of Platy- 

 rrhini and Katarrhini. This vagina is less original than that of Platy- 

 rrhini, as in man the piercing-process extends also over the M. trans- 

 versus, but because the piercing of the M. obliq. int. by the M. rectus 

 is not yet complete, the sheath of man is at the same time more 

 original than that of Katarrhini. 



The linea somicircularis Douglasii is the line along which the 

 transversiis aponeurosis modities its direction with regard to the 

 M. rectus ; it is formed by the last fibres of the M. transversus, 

 which proceed behind the M. rectus towards the linea alba (with 

 man the last libres of the posterior layer of the aponeurosis of the 

 M. ()lili(|. int. take moreover part in the formation of the linea). 

 The possession of a linea Douglasii does conse()uently mean, that 

 the |)icrciiig-process that takes place in the sheath of the M. rectus, 

 has adxanccd so much, that also the M. transversus is pierced in 

 the caudal part by the M. rectus. By this explanation a new light 

 is thrown on the dark question about tlie signitication of the linea, 

 a question, that, notwithstanding the ditferent hy[)0lhescs that have 

 been suggested, in order to explain this phenomenon in the posterior 

 lamella of the rectal sheath, has not yet found a satisfactory solution. 

 We need by no means be astonished at this fact, as, indeed, all 

 investigators, who have hitherto occupied themselves with this quest- 



