134 



COMPARATIVE ANATOMY 



in the adult stage, the integument and the pharyngeal hning carry on all 

 of the respiratory interchange. Accordingly capillaries penetrate to some 

 extent into the epidermis and pharyngeal epithehum, thereby bringing 

 blood into closer relation to the external medium. 



Many epithelia, although "simple" in the sense of being only one cell 

 thick, are not the ideally simple tissues of the definition (page 126), consti- 

 tuted of cells all "aUke in their internal differentiation." Among the 



Stratum 

 comeum. 



Stratum 

 lucidum. 



Stratum 

 granulosum. 



Stratum 

 germinativuiti. 



Corium 

 (Tunica 

 propria.) 



Fig. 95. — Epidermis from the sole of the foot of an adult man. Section perpendicu- 

 lar to surface of skin. External to the stratum germinativum, the strata show successive 

 stages in the production of the stratum comeum. X360. (From Bremer, Text-book 

 of Histology.) 



special functions of an epithehum are the following: (i) production of a 

 superficial covering of protective, non-living, mechanically resistant sub- 

 stance; (2) production of special secretions such as mucus; (3) reception of 

 external stimuH; (4) provision for motile activity. Two or more of these 

 functions may be carried on by one "simple" epithehum or by a stratified 

 epithehum. Within the epithelium, then, cells will exhibit differentiation 

 of as many types as there are functions. 



(i) Most epitheha produce at the free surface a protective covering 

 which ranges from a very thin and dehcate cuticula, such as occurs on the 

 epidermis of a small fish, to the massive horny layer on a large reptile or 

 the hoof of a horse. 



