CHAP, ii.] THE CATS GENERAL FORM. 21 



time cartilaginous. These cartilaginous ends ossify subsequently, 

 but long continue distinct from the median part and are called 

 epiphyses (Fig. 6), which only unite with the rest of the bone when 

 the animal has attained maturity. Epiphyses are often developed 

 at the ends of any projecting pieces of bone or " processes." 



A bone may thus be developed from more than one point, i. e., 

 from several " centres of ossification," the respective growths from 

 which ultimately unite to form one whole. A continuation of the 

 same process may fuse together even entire bones which have for a 

 time remained separate and distinct. 



The most external layer of the skin consists of yet another 

 substance, which is known as epithelial tissue, and which is very 

 distinct in nature from connective tissue or the elastic cartilaginous 

 or osseous modifications of connective tissue. 



8. Such are the substances or tissues of which the cat's skeleton 

 is in its entirety composed. 



That skeleton is naturally divisible into two parts : 



(a.) The external, peripheral skeleton, often called the Exo- 



SKELETON the skin and its appendages. 

 (b.) The internal central skeleton, often termed the ENDO- 



SKELETON. 



9. The EXTERNAL SKELETON of the cat is made up of its skin, 

 with the hair which coats it, the claws, and also the teeth. No 

 cartilage or true bone enters into its composition. 



The skin of the cat, like our own skin, consists of TWO LAYERS : 

 an external layer, devoid of nerves and blood-vessels (and conse- 

 quently of feeling), and a deeper layer, which is supplied with both 

 nerves and blood-vessels, and is highly sensitive. The external 

 layer is called the epidermis, the deep layer is called the dermis. 

 At the lips the external layer visibly changes in texture, and inside 

 the lips and mouth it becomes soft and moist, and is termed mucous 

 membrane. This, however, is a mere modified continuation of the 

 external skin. The superficial layer of inwardly reflected skin is 

 termed the epithelium, which is thus but a modified epidermis, and 

 the common term ECTERON is applied to both epidermis and epithe- 

 lium, as the term ENDERON is applied to the deeper or dermal layer 

 (L e., the dermis) wherever situate. 



10. The Epidermis is an EPITHELIAL TISSUE, and consists of 

 numerous superimposed layers of epithelial cells, of which those near 

 the surface are flattened into scales, while the deeper ones are more 

 and more rounded, the deepest even assuming a vertically ^elongated 

 form. As the epidermis is worn away from the surface in minute 

 fragments, newer cells rise successively from below (to replace those 

 lost) from a layer of structureless substance which connects the epi- 

 dermis with the subjacent dermis. In this layer minute particles 

 (nuclei) arise and gather round them spheroidal portions of the sub- 

 stance itself, thus forming cells which subsequently multiply by spon- 

 taneous division or fission, the process commencing with the division 

 of the nucleus of each cell. The deeper strata of epidermis contain 



