86 CHORD ATE ANATOMY 



Most vital functions involve the surface between protoplasm and 

 the medium immediately external to it. Food enters from without. 

 Respiratory gases pass in and out. Waste is expelled from the surface. 

 Special secretions are produced at the surface. External forces impinge 

 upon the surface. Further, most of the organs of the adult animal are 

 hollow. They contain something or they convey something — food, 

 air, blood. Even such organs as the liver and pancreas, upon casual 

 inspection apparently quite solid, are minutely hollow. Muscles, how- 

 ever, are solid. Connective and skeletal tissues may form bulky solid 

 masses — solid, that is, except insofar as they are penetrated by blood 

 vessels. Bone may contain cavities, but these cavities have a merely 

 passive mechanical significance. The occupation of bone cavities by a 

 blood-forming marrow makes advantageous use of what might otherwise 

 be mere waste space in the animal, but this marrow tissue has no direct 

 relation to the skeletal function of the bone. Such nervous organs as 

 brains, ganglia, central nerve cords, and nerves need not be hollow and 

 ordinarily are not. 



Every surface of the animal, whether apposed directly to the external 

 medium or to some internal cavity, is a critical region. It is a surface 

 on the one side of which is living substance while on the other side of it 

 may be food, water, air, blood or something else between which and the 

 protoplasm is being carried on some vitally necessary activity — digestion, 

 respiration, absorption, secretion, excretion, diffusion. Or it may be a 

 surface at which the underlying protoplasm deposits a protective non- 

 living substance. 



Provision for the adequate carr3dng on of these essential and diverse 

 surface activities can be afforded only by the presence of a superficial 

 membrane constituted of living material and specialized appropriately 

 for the functional requirements of the particular surface. Consequently, 

 with very rare exceptions, every free surface of an animal, external or 

 internal, is the surface of a more or less specialized cellular la_ver, an 



EPITHELIUM. 



EPITHELIAL TISSUES 



Epithelia are tissues of primary importance. They are, in double 

 sense, the most primitive of tissues. The smaller simpler coelenterates 

 consist merely of an outer and an inner epithelium. The gastrula of 

 animal embryos consists of two epitheha. It is evident, then, that 

 epithelium provides for all animal needs, and therefore all-epithelial 

 animals may and do exist. 



The outer layer of the vertebrate gastrula, while it is the source of 

 various structures which attain a deeper position, otherwise persists as 

 the epidermis which is the external epithelium of the adult body. The 



