GENERAL STUDY OF TYPICAL CHORD ATES 27 



with protruding lips. On the dorsal side just in front of the base of the tail 

 will be seen by lifting up the tail coverts or, on the plucked bird, a papilla, the 

 opening of the uropygial gland, from which the bird obtains oil for preening 

 its feathers. 



M. EXTERNAL ANATOMY OF A MAMMAL 



For this purpose either the cat or the rabbit may be used. Both are 

 mammals belonging to the class Mammalia. The rabbit is a rodent, the cat a 

 carnivore. 



1. Body form and skin. — The body is divisible into head, neck, trunk, and 

 tail. Its proportions depart somewhat from the typical vertebrate form in the 

 larger size of the head and the reduction of the tail, the latter feature particularly 

 noticeable in the rabbit. The body is clothed with closely set hairs, forming a 

 covering of fur, characteristic of mammals. Upon the head are a number of 

 especially long and stout hairs, the whiskers or vibrissae, which serve as important 

 organs of touch. 



2. Head. — The large size of the head is due to the great development of 

 the brain within it. The head may consequently be divided into an anterior 

 facial region, in front of the eyes, and an enlarged posterior cranial region. 

 The mouth is provided with well-developed lips. The upper lip is cleft in the 

 center, deeply so in the rabbit, exposing the incisor teeth. The external nares 

 are large and elongated, overhung by the mobile nose. The eyes have upper 

 and lower lids and a nictitating membrane. The latter should be sought with 

 a forceps in the anterior corner of the eye and drawn over the eyeball. The 

 ears are provided with a long and flexible external fold, the pinna, which springs 

 from the rim of an opening, the external auditory meatus, which descends into 

 the interior of the skull. Pinna and meatus constitute the external ear. 



3. Trunk. — The trunk is divisible into an anterior chest or thorax, supported 

 by the ribs, and a posterior abdomen. On the ventral surface of the trunk are 

 four or five pairs of teats or nipples in female specimens; they are the openings 

 of the milk glands or mammary glands. The trunk bears two pairs of limbs, 

 which consist of the same parts as other vertebrate limbs. The upper section 

 of each limb — that is, thigh or upper arm — is more or less included in the trunk. 

 The limbs terminate in clawed digits, five in front, four behind, the first hind 

 toe being absent. The claws of the cat and its relatives are retractile. 



The limbs have undergone a marked change from the primitive position. 

 Instead of extending laterally from the body they project ventrally and are 

 elongated, so that the body of the animal is carried high above the ground. This 

 change has involved a rotation of 90 in each limb. The hind limb is rotated 

 forward so that the original dorsal surface is anterior and the toes point forward. 

 The knee or joint between the thigh and shank is likewise directed anteriorly. 

 The fore limb, on the contrary, has rotated posteriorly, so that the original 



