Ryder.] (Jan. 3, 1890. 



the entrance of the optic nerve ; it extends back and its origin is lost in the 

 connective tissue of the lachrymal gland. No definite account of the 

 recti muscles or of the oblique muscles can be given here. All that my 

 sections disclose is the fact that muscles which are apparently the homo- 

 logues of the recti are inserted upon the sclerotic nearly as far forward as 

 the ciliary region. These muscles, like the choanoid, pass backward to 

 arise from the connective tissue of the lachrymal gland with which they 

 blend and in which they become lost, or they join a relatively thick 

 muscular tunic composed of voluntary muscular fibres which invests 

 nearly the whole lachrymal gland. 



This muscular investment of the lachrymal gland suggests that the 

 function of such a muscular apparatus is to compress the tear gland and 

 force its secretion over the eye-ball, and to thus wash away any dirt which 

 may find its way under the very much reduced eye-lids, the opening in 

 which is scarcely half a millimetre wide. 



The foregoing brief sketch of the anatomy of the eyes of one of the 

 commonest of our American shrew-mice suggests much in the way of 

 further study. The remarkable and apparently voluntary mechanism for 

 compressing the tear gland is evidence distinctly against the conclusion 

 as respects at least our North American shrews, reached by Mr. Darwin 

 in regard to the Tucu-tuco or Ctenomys of South America, in which case he 

 suggests that the repeated irritation and inflammation of the eyes of these 

 burrowing rodents, due to the dirt or sand which found its way beneath 

 the lids, would aid in rendering the eyes inefficient, and in the course of 

 generations abortive, as they are sometimes found to be. It is clear that 

 if the interpretation of the function of the muscular investment of the 

 tear gland in Blarina here suggested is correct, that in this case, at least> 

 there is a direct and very special structural provision by which irritation 

 from the presence of sand or dirt in the eyes, as a consequence of a bur- 

 rowing habit, may be guarded against in the most efficient manner con- 

 ceivable. 



That the eye of Blarina, as a whole, has suffered from degeneration 

 may be inferred with certainty from the diminutive size of the eye-ball 

 and optic nerve, and the evidence furnished by the muscles suggests that 

 while the eye-ball is no longer rotated in precisely the manner which 

 obtains in other forms, it is clear that there are ocular muscles, ancl that 

 theeye is capable of adjustment for the direction of vision, though it is 

 evident that the muscles which effect such an adjustment no longer arise 

 directly from the skull, as in all other normal forms of the eye of verte- 

 brates. The extra tunic of voluntary muscular fibres investing the lach- 

 rymal gland seems to be something which has been superadded to the 

 optic apparatus of Blarina, which, like the relatively large lachrymal 

 gland itself, is really an indication of specialization to meet the require- 

 ments of special conditions of life. 



