12. STUDIES IN LIFE AND SENSE. 



the " panniculus " are also to be found. In the proportion of about 

 three per cent in upwards of 600 bodies examined, Professor Turner 

 tells us that a muscle of man's trunk, usually regarded by anatomists 

 as of ordinary type, is really a fragment of the great " skin-muscle." 

 Again, a curious muscle in man, known as the palmaris brevis, lying 

 imbedded in the fat of the inner or little finger side of the palm of 

 the hand, and which passes to be attached to the skin covering the 

 ball of the little finger, is to be regarded as a relic of the skin-muscle 

 of lower life. This muscle also occurs in the lemurs, and is found 

 even in the kangaroo order. It may therefore represent a very early 

 development of the "skin-muscle," which became separated from 

 the great bulk of that structure long before the other portions assumed 

 a distinct individuality. So far, therefore, as man's muscular arrange- 

 ments are concerned, he may be said to exhibit no peculiarities which 

 cannot be accounted for on grounds compatible with the belief that 

 his most characteristic belongings represent either special develop- 

 ments, or on the other hand degenerations, of the fibres of lower forms. 

 From the " skin-muscles " and face to the eye is but a brief step ; 

 and in certain structures connected with the organ of sight in man 

 we may discover other links relating man to lower life. The " third 

 eyelid " of animal life has already been cursorily alluded to. This 

 structure is the " nictitating membrane " of comparative anatomists. 

 As low down in the vertebrate series as the sharks, this curious 

 structure is to be found ; amongst the frogs and reptiles it is far from 

 uncommon ; and in birds it assumes a high prominence amongst the 

 belongings of the eye. Amongst the quadrupeds, it occasionally 

 exhibits a tolerable development. In the walrus, for instance, it is 

 seen in perfection, whilst amongst the lowest mammals the kangaroo 

 order and that of the " duck-billed water mole " of Australia it is 

 specially developed. Of higher quadruped life as a whole, the " third 

 eyelid," however, cannot be said to be characteristic. In the birds, 

 in which the " third eyelid " is specially developed, this structure 

 exists as a membrane which sweeps across the surface of the eyeball 

 from the inner to the outer side, somewhat after the fashion of a 

 shutter. In many birds, this eyelid is of a white hue, in others it is 

 transparent. Two special muscles assist its movements. These 

 muscles pull the eyelid over the globe of the eye, and upon their re- 

 laxing their action, the membrane returns through its own elasticity 

 to the inner corner of the eye-cavity, where it lies folded until again 

 called into use. In man and the apes, no third eyelid is developed ; 

 a feature in which they agree with the great bulk of the quadrupeds. 

 But when the eyes of man and his nearest allies are carefully ex- 

 amined, a curious, little, half-moon-shaped fold (plica semilunaris} is 

 seen to exist at the inner corner of the eye-border. Between the 

 two layers of which this little fold is composed, there is developed a 



