THE MONOTREME EYE 665 



and by Gresser and Noback. None of these accounts is entirely accurate 

 — all incorporate particularly serious errors in regard to the shape of the 

 globe (which is 'avian' only when collapsed) and the presence of a 

 ciliary muscle (which is wholly lacking, though two of these authors 

 describe it as having the same three types of fibers — meridional, 'radial', 

 and circular — as the ciliary muscle of man). The ensuing descriptions 

 are based upon preparations of Tachyglossus and Ornithorhynchus made 

 by Kevin O'Day, and upon correspondence with him. Statements of 

 earlier workers which happen not to be refuted by O'Day's splendid 

 material are also incorporated.* 



Ornithorhynchus has an excellent nictitating membrane. Tachyglossus 

 has none; but both genera have retractor bulbi muscles. The lids are 

 plump and small in both, and in Tachyglossus are closed by swinging 

 rather than by sliding. Small Meibomian glands, still with relation to 

 hair follicles, are present in Ornithorhynchus. These may be orimentary 

 (see p. 40) ; but the same situation occurs in one placental, the hedge- 

 hog (Erinaceus) . They are lacking in Tachyglossus and Zaglossus. Like 

 most Sauropsida, Tachyglossus has a tarsus in the lower lid only, while 

 Ornithorhynchus has one in each lid. Both genera are supposed to have 

 both lacrimal and Harderian glands (but Kolmer found only serous 

 glandular tissue in Zaglossus) . The adnexa in Ornithorhynchus thus 

 show no specialization for the amphibious life of the animal. In fact, 

 those of the echidnas exhibit rather more reduction, which seems largely 

 explained by the presence in those forms of a keratinization of the 

 corneal epithelium, no doubt in adaptation to the ant-eating habit (as 

 in armadillos and aard-varks) . 



In the arrangement of the superior oblique muscle, the monotremes 

 are wholly 'mammalian'. In the echidnas there is a slip which runs from 

 the old sub-mammalian origin (on the anterior nasal orbital wall) to 

 an insertion on the globe; but merging in this same insertion is a second 

 slip, muscular almost to the globe, which comes through a pulley from 

 an origin only a few millimeters anterior to the deep point-of-origin of 

 the four recti. The duck-bill has only this long portion, and moreover 

 has it as in higher mammals, /'. e. originating with the recti and becoming 



*Dr. O'Day has been trying for several years to find time to prepare a monograph on the 

 eyes of the monotremes. When this does finally appear it will greatly extend, and no doubt 

 partially contradict, the present treatment. In the meantime, because of the slowness and 

 uncertainty of communication with Australia, the writer has made bold to discuss O'Day's 

 findings without seeking his permission — they seem much too important to be left out of 

 this book. 



