THE SKIN AND SENSE ORGANS 301 



one-third or one-quarter that of the majority of the Anura. It is dull 

 white in colour and is not covered by anything, nor is there pigment 

 nor differentiation of any kind, nor the slightest trace of any vessels, 

 either hyaloidean or discoidal.' In this last feature Salamandra differs 

 markedly from the Frog. Another difference between these two 

 animals lies in the absence, in the Salamander, of any trace of the 

 vertical pigmented stripe which extends from the lower edge of the 

 pupil to the margin of the iris in the Anura. Johnson supposes that 

 this stripe, which is always associated with a groove and which some- 

 times extends to the upper portion of the iris also, is the result of the 

 mechanical strain of the iris muscles, and is phyletically the oldest 

 condition, and that the round pupil of the Salamander represents the 

 highest stage of development. The same author finds considerable 

 accommodating powers, but no movement of the eye itself other than 

 that of retraction. This last feature is not surprising in view of the 

 relatively weak development of the ocular muscles other than M. 

 retractor bulhi. 



Quite recently Dr. Ida C. Mann (1931) has made an exhaustive 

 comparative study of the iris pattern in vertebrates, and notes that 

 in the Salamander the arteries enter the iris from below and on the 

 temporal side, while in some specimens the inferior iris artery is a 

 branch of the temporal and does not enter separately. This latter she 

 thinks is the more typical Urodele arrangement. The arteries break 

 up irregularly round the pupil and the blood drains away by a few 

 radial veins, not definitely superficial to the arteries and not arranged 

 with anything like the regularity found in Reptiles. The pigmenta- 

 tion of the iris is entirely brown. 



2. Glands of the Orbit. 



Apart from histological details there is not much that can be added 

 concerning the structure of the eye itself. It is supplied by a pair of 

 ciliary nerves, a superior (p. 140) and an inferior (p. 140), while the 

 bulbus oculi is attached to the wall of the skull by a strong fibrous 

 sheath surrounding the optic nerve^ the whole forming the opticpeduncle. 

 Within the orbit, beneath the bulbus oculi, is a mass of glandular 

 tissue which shows a tendency to become differentiated into two 

 sections. The anterior region is the Harderian gland^ and the pos- 

 terior the lacrimal gland. In Salamandra the separation of the Har- 

 derian portion in the anterior angle of the orbit from the remainder 

 of the gland may be partial or complete. In Triton there is no such 

 separation (Sardemann, 1887). Hence this animal may be regarded 

 as exemplifying the primitive condition, while the Salamander shows, 



