RAT, A REPRESENTATIVE MAMMAL 109 



in part by cartilage. It is nearly devoid of hair. At the base of 

 this is an ear opening, the external auditory meatus, which leads to 

 the interior of the skull and to the tympanic membrane. The head 

 region is well supplied with the endings of all the five principal 

 senses : sight, hearing, smell, touch, and taste. 



The trunk is the large portion of the body posterior to the head. 

 The relatively short cervical region, or neck, joins the head and 

 trunk. Behind the cervical region is found the chest or thoracic 

 region and following this is the abdominal region. The latter may 

 be divided into the lumbar region (small of the back) and sacral or 

 hip region. Continuing posteriori}^ from the trunk is the tail or 

 caudal region. Attached to the sides of the trunk are the two pairs 

 of limbs or appendages. The fore limbs or arms are composed of 

 the following parts in order: upper arm or brachium, forearm or 

 antebrachium, wrist or carpus, and hand or manus with its fingers 

 or digits. There are four well-developed digits and a vestigial thumb. 

 Beginning at the body each hind limb consists of thigh, shank, ankle 

 (tarsus), and foot (pes) with its plantar region (sole) and five digits 

 (toes). There are claws on the digits of both fore and hind limbs. 

 Since the rat walks with the soles and palms of the feet in contact 

 with the ground, it is said to be plantigrade in its gait. Animals 

 like cats and dogs which walk with only the lower sides of the 

 digits on the ground are digitigrade while animals like horses and 

 cattle which walk on the tips of the digits are unguligrade. 



Along the ventral side of the abdomen are five pairs of nipples or 

 teats in the albino and six pairs in the Norway rat. In the perineal 

 region, just beneath the base of the tail is located the anus or opening 

 through which the fecal matter is discharged from the digestive 

 tract. Situated also in the perineal region are the external genital 

 organs. In the male the scrotum is the conspicuous swelling ventral 

 to the anus. The two testes (sex glands) are usually held inside the 

 scrotum, but sometimes they lie up in the body cavity instead, in 

 which case they are poorly developed and much reduced. The ex- 

 ternal opening of the male urogenital system is in the midventral 

 line near the anterior end of the scrotum. The prepuce (foreskin), 

 which is hairy externally, forms the aperture and surrounds the 

 distal end or glans of the penis. The latter is withdrawn. In the 

 perineal region of the female there are three openings: the anus, 

 ventral to the base of the tail; the vaginal orifice (exit from female 



