666 MAMMALS 



tendinous before reaching the pulley, with the latter chondroid rather 

 than soft as in the echidnas. This seems too strong a similarity to the 

 higher mammals to be dismissed as a coincidence by those who consider 

 the monotremes to have originated from a separate reptilian stock. It is 

 not certain what called forth the elongation of the mammalian superior 

 oblique. Such an elongation may have occurred twice. In this connection, 

 it would be nice to know whether the optic chiasmata of the mono- 

 tremes are only partially decussated. Both types have wide binocular 

 fields, that of the echidnas being projected forward and that of the duck- 

 bill largely upward. 



The eyeball is usually figured with a short axis and a pronounced 

 circumcorneal scleral sulcus, both of which are collapse-artefacts. Cor- 

 respondingly, its shape has most often been called 'avian',* Actually, 

 the eyeball is everywhere convex and is spherical in all monotremes. This 

 sphericity, so reminiscent of the snakes, has the same basis — a total 

 disappearance of the ancestral scleral ossicles (Fig. 194a) . 



The eyeball of Tachyglossus is eight or nine millimeters in diameter, 

 that of Ornithorhynchus about six. In all monotremes the sclera con- 

 tains the cartilage cup with which we have become so familiar in pre- 

 ceding chapters. In Tachyglossus the cartilage is 27[J, thick in the region 

 of the optic nerve, 14[X thick near its sharp anterior lip. In Z.aglossus 

 (a larger animal) it averages 160|i in thickness. In the duck-bill it is 

 even thicker fundally (400(l) but tapers to 25 (X near its knife-edge 

 termination. The cartilage reaches to the posterior ends of the ciliary 

 processes in Ornithorhynchus, but stops opposite the ora terminalis in 

 Z.ctglossus and a little behind the ora in Tachyglossus. An outer layer 

 of fibrous scleral tissue about equal in thickness to the cartilage (but 

 only 96 [A in ^aglossus) , continues forward (receiving an addition which 

 replaces the cartilage) through a zone formerly occupied by the scleral 

 ossicles, and blends with the substantia propria of the cornea. In Tachy- 

 glossus at least, an outer fraction of the substantia propria is easily seen 

 to be continuous with the conjunctival corium or 'episcleral' connective 

 tissue. A loose layer of episcleral blood vessels, from which capillaries 



*And all the sauropsidoid internal features are likewise called avian by those who are 

 familiar with their occurrence in birds but ignorant of their occurrence also in the reptiles. 

 Attempts to derive the monotreme eye from the avian, and coy insinuations that the two 

 eyes are identical through convergence (justifying the 'bill', webbed feet, spurs, and egg- 

 laying habit of the platypus), are naive in the extreme; but they continue to be made. 



The astute Franz indicates in several places that he suspects that the 'avian' form of the 

 usual preserved echidna eye is a result of collapse. O'Day finds that this collapse occurs 

 very readily in both Tachyglossus and Ornithorhynchus. 



