324 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Type of the Vertebrata, — Fells domestica (Figs. 139-142). 



The cat is a four-footed mammal with the protective skin 

 largely covered with soft hair and with an internal supporting 

 skeleton of bone. Its organs of offense and defense are sharp 

 carnivorous teeth and curved claws. 



The elastic skin, which is united to the underlying flesh 

 by a connective tissue, is composed of a deeper layer, the dermis, 

 or corium, and an outer layer, the epidermis. As the super- 

 ficial cells of the latter are continually drying up and falling 

 off, new cells form beneath. The claws and hair are special 

 modifications of the epidermis. The oil-glands, the product 

 of which keeps the surface of the skin and the hair soft, are 

 developed in the dermis, and each usually opens into a hair 

 follicle, — a sac-like sheath lodging the base of the hair. The 

 sweat-glands, whose function is in some mammals to aid in 

 the maintenance of a uniform body temperature, are in the 

 deepest part of the dermis, or even in the tissue beneath, with 

 openings at the surface. They are of comparatively slight 

 importance in the Carnivora, the order to which the cat belongs. 



Skeleton. — - The cat's body is supported by an internal 

 framework of bone and moved by muscles attached to their 

 outer surfaces. There are about 230 separate bones ; these 

 are fewer in the old than in the young, owing to the union of 

 some bones later in life. Bone is composed of about one-third 

 organic matter (gelatine and blood-vessels) and two-thirds in- 

 organic matter (lime-phosphate about fifty and lime-carbon- 

 ate ten per cent). Each bone is covered completely, except 

 on its articulating surfaces, with a membrane, the periosteum ; 

 this serves to renew the bone when injured. 



The principal portion of the skeleton is the backbone ; to 

 it are attached either directly or indirectly all the other bones 

 of the body (see Fig. 139). To it anteriorly is attached the 

 skull, — the brain-inclosing box, to which the upper jaw (max- 

 illa) is solidly joined in front and the lower jaw (mandible) 

 attached by muscles and ligaments below, while posteriorly are 



