88 The Vertebrate Organ Systems 



3-1-3-1 . , , 1-0-0-3 

 cat's formula is . The rat has the followinp^ formula: . 



3-1-2-1 1-0-0-3 



The space where no teeth are present is known as the diastema. 



In most mammals, the individual has but two sets of teeth to serve 

 him throughout his lifetime. The first set, known as the "milk teeth," 

 consists of an incomplete set. After the last "milk molar" erupts, the 

 permanent molars appear. Before these are completely in place, the 

 "milk teeth" are gradually replaced by the permanent incisors, canines, 

 and premolars. 



Most teeth do not continue growth once they are in place. There 

 are exceptions to this generalization. The incisors of rodents and 

 the tusks of pigs grow indefinitely. It is only by gnawing that the 

 ends of these teeth are kept short and sharp. 



Horns. — Horns are found only among the hoofed animals and 

 are of several types. The simplest type is that known as the keratin- 

 fiber horn. This type is found among the rhinoceroses and is entirely 

 epidermal. It is formed of hairlike keratin fibers cemented together 

 into a compact mass. 



Antlers found among the deer consist entirely of a bony outgrowth 

 from the skull and, properly, should not be included in this discussion. 

 At first, they are covered with a hairy skin, "the velvet," which ulti- 

 mately dries and rubs off. This represents the only integumentary 

 portion of these horns. The horns are shed at the end of the second 

 year before the mating season. Following this, the skin grows over 

 the wound, and a new pair of antlers starts growing. 



The horns of cattle are known as hollozv horns. These are strictly 

 epidermal structures which are not shed. The germinativum layer con- 

 stantly restores the horn as it wears away. The horn thus is a hollow 

 corneal structure which fits over a core of bone attached to the skull. 



The last type of horn is that known as the pronghorn. These are 

 found in the pronghorn antelope. These horns consist of permanent 

 bony antlers covered with a sheath of horny integument. This latter 

 part is shed periodically and then replaced. The bony core, however, is 

 permanent. 



Feathers. — Feathers are found in but one class of vertebrates, 

 the birds. For these animals, the feathers serve a number of very use- 

 ful purposes. They are excellent insulators, helping to maintain the 

 high body temperature characteristic of these animals; they are ex- 



