OF THE EYE MUSCLES IX MAKSUPIALS. 303 



also from the desci-iption of the development of the veins of the 

 head in reptiles by Grosser and Brezina ('95). 



The Eye Muscles of the Adult. 



Adult specimens of Trichosurus and Dasyurus were examined, 

 and the usual eye muscles, including a well-developed m. retractor 

 bulbi, were found to be present. In 1902, Corning ('02) 

 described in some detail the eye muscles of the cat, and his 

 account is generally applicable to the recti and oblique muscles 

 of the marsupial. The only noteworthy difference is in regard to 

 the form of the m. rectus externus which in marsupials has a 

 double origin, the two portions uniting to have a common 

 insertion on the eyeball. 



The m. retractor bulbi shows considerablevariation in different 

 mammals. When present, it always arises further caudally than 

 the m. rectus externus but enters the orbit together with the 

 mm. recti a,nd the m. obliquus supeiior, and its insertion in the 

 bulbus lies nearer the optic nerve than the other muscles. 

 According to Motais ('87), who has studied this muscle in many 

 mammals, the m. retractor bulbi attains its maximum develop- 

 ment in ruminants ; he also observed its relatively large size in 

 the opossum. He further states, p. 56 : " Le muscle choanoide 

 pent offrir un on plusieurs interstices celluleux qui le divisent en 

 deux ou plusieurs parties. Ces interstices sont larges dans les 

 carnivores et separent le muscle en quatre faisceaux bien distincts. 

 Ordinairement, les lignes de separation sont moins nettes ; dans 

 les solipedes et les ruminants, les bords des deux divisions du 

 muscle s'envoient reciproquement des fascicules .... Dans le 

 pore, on ne trouve qu'un seul interstice assez large an milieu du 

 muscle droit inferieur." Similar conditions to those in the pig 

 a.re found in rodents ; the same author, in his description of the 

 rabbit, p. 206, says : " II [le muscle choanoide] forme un cone 

 regulier qui n'est interrompu que par un seul interstice celluleux 

 situe au-dessous du muscle droit superieur. Cet interstice se 

 prolonge jusqu'a I'insertion bulbaire du nerf optique et le long du 

 nerf lui-meme, jusqu'au trou optique." In the cetaceans (Weber, 

 '86) the same variations occur; here again, the muscle maybe 

 present as an almost complete circula.r sheet surrounding the 

 optic nerve or it may be divided into four parts, each moi'e or less 

 united by connective tissue. 



In Trichositrus and Dasyt^nts, as in other mammals, the 

 m. retractor bulbi has a more deeply seated origin than the other 

 muscles. It arises from the basisphenoid and passes foi'wards to 

 enter the orbit between the two portions of the m. rectus externus. 

 Surrounding the optic nerve, it extends outwards as a muscular 

 sheet which gradually increases in circumference up to its insertion 

 round the inner side of the eyeball within the recti muscles. 

 Although appearing as a completely closed cone, the two edges of 

 the sheet are not fused along the anterior side of the nerve just 

 behind the m. rectus internus. 



