230 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



up in its anterior angle. A little way behind and below the eye 

 is a rounded auditory opening concealed by feathers. It opens 

 into a short tube, the external auditory meatus. 



The trunk is somewhat boat-shaped, presenting a prominent 

 ridge along the middle of its ventral side, except for a small 

 posterior area where soft abdominal walls can be left. A large 

 transverse cloacal aperture with swollen edges, opens ventrally at 

 the base of the tail. Upon the dorsal surface of the latter 

 structure there is a small papilla, upon which the oil-gland opens 

 (Fig. 70, u.gt). 



The fore-limb (cf. Fig. 69) is divided into upper arm, fore- 

 arm, and hand (brachium, antebrachium, and manus), which are 

 of about equal length, and folded closely one upon another when 

 not extended for flight, but, when so extended, approximating to 

 the primitive position. The manus is tridadyle, but the small 

 thumb (pollex} is the only digit well-marked externally. It bears 

 a small tuft of feathers, the bastard wing. Eleven of the remiges 

 are attached to the hand. These are the primary quills. The 

 remaining eight remiges, secondary quills, are attached to the fore- 

 arm. 



The hind-limb (cf. Fig. 69) is divided into thigh, leg, and 

 foot (femur, crus, and pes). The last is made up of a cylindrical 

 tarsometatarsus, about as long as the thigh (the leg is much 

 longer), and four digits, of which the first, or great toe (hallux), 

 is 'directed backwards, the others forwards. The foot is covered 

 by overlapping scales, and the digits are terminated by claws. 



Position of Body. The directions assumed by the divisions of 

 the fore-limb have already been mentioned. In the standing 

 posture the thigh, leg, and tibio-tarsus are arranged in a longi- 

 tudinal vertical plane with the knee directed forwards and the 

 mesotarsal ankle-joint backwards. The great toe is also turned 

 backwards, and owing to its free metatarsal is more mobile 

 than the other forwardly directed digits. 



2. The extremely thin skin is divided into epidermis and 

 dermis, the former possessing horny and deeper layers. The 

 scales and claws are made up of coalesced epidermic cells, and 

 the feathers are also modifications of this layer. A feather 

 consists of a central stem, the proximal part of which is a hollow 

 quill, and the distal part a solid shaft, forming the central part 

 of the expanded vane, the rest of which is made up of a row of 



