PROTECTIVE MECHANISMS OF THE EYEBALL 99 



examination of Tenon's capsule in the orbit of man and 

 monkeys convinces me that it is the degenerated represent- 

 ative of the retractor bulbi as surely as the little pleat of 

 mucous membrane, known as the semilunar fold, is the 

 vestige or remnant of the third eyelid." 



Motais 50 has found the capsule of Tenon constructed in 

 a uniform manner in all classes of vertebrates; it presents, 

 he says, only slight variations due to differences in the 

 arrangements of the ocular muscles. He has published 

 pictures showing its arrangements in the eyes of a horse 

 and of a cow, in which also the choanoid muscle is present. 

 I have found it present myself in dissections I have made 

 of an antelope (Fig. 22) and of a sheep. It ensheaths, and 

 stretches between, the recti muscles, and forms, moreover, 

 a sheath to the choanoid muscle itself. Inasmuch, then, as 

 Tenon's capsule is found in animals in whom the choanoid 

 muscle is present, it would seem impossible to regard it as 

 a vestigial remnant of the muscle when absent. 



Lindsay Johnson 3 states that he met with a case of an 

 obvious membrana nictitans in a youth which was capable 

 of slight movement and extended in a crescent form nearly 

 as far as the cornea. He does not say, however, whether 

 this movement of the membrane was effected by any accom- 

 panying displacement of the eyeball backward. In 1912, 

 S. E. Whitwall 51 recorded an instance of a retractor bulbi 

 muscle present in both orbits of a man, aged fifty years. 

 He states that vestiges of it are also met with in some monkeys 

 (e. g., Macacus rhesus). 



Though the evidence of any vestigial remains of the 

 choanoid, or retractor bulbi muscle, is very doubtful, the 

 presence of vestigial remains of Gegenbauer's orbital muscle, 

 or, as I suggest it might be well termed, "the protrusio 

 bulbi muscle," is clear and conclusive. 



