EXTERNAL CHARACTERS. 337 



3. The eyes are large, and placed at the sides of the head. 



Each eye is protected by an upper and a lower eye- 

 lid, fringed with scanty eyelashes ; and by a third 

 eyelid, or nictitating membrane, a hairless fold of 

 opaque white skin, which lies within the other eye- 

 lids, and can be pulled across the eyeball from the 

 anterior angle or canthus. 



4. The vibrissas, or whiskers, are long stiff tactile hairs, at 



the side of the snout and above and below the eyes. 



5. The external ears, or pinnae, are very large and freely 



movable. Each is folded longitudinally, and is sup- 

 ported by cartilage at its base, where it surrounds 

 the external auditory aperture. 



B. The Trunk. 



This is divided into an anterior portion or thorax, stiffened 

 laterally by the ribs, and a posterior part or abdomen. 



Feel through the skin, and determine the positions of the 

 vertebral column, the ribs, and the sternum. 



1. The anus is a small median aperture beneath the base of 



the tail. 



2. The perinseal pouches are a pair of hairless depressions 



of the skin, in front of and at the sides of the anus : 

 into them open the ducts of the perinseal glands, the 

 secretion of which has the strong smell characteristic 

 of the rabbit. These glands should be removed at 

 once if the rat is under dissection. 



3. The urino-genital aperture is situated in both sexes in 



front of the anus. 



a. In the male the aperture is placed at the end of the 



long cylindrical penis, which can be retracted 

 within a loose sheath of skin, the prepuce. 



b. In the female the opening, or vulva, is slit-like^ and 



larger than in the male ; and its anterior margin 

 is produced into a smajl fleshy process, the 

 clitoris, which corresponds to the penis of the 

 male. 



