128 COMPAEATIVE ANATOMY 



double wall encloses a cavity which is the primitive enteron. Very 

 soon the gastrula adds a mesoderm, a third layer, and for some time 

 thereafter all of the embryonic material continues to be disposed in layers 

 of cells more or less widely separated by spaces. There is no important 

 departure from this arrangement until the myotomes begin to thicken, 

 preparatory to forming muscle, and mesenchyme cells begin to aggregate 

 around the notochord and neural tube preparatory to forming skeleton 

 — and by this time the circulatory system is beginning to develop. 



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. The essential liver tissue consists of tubules 

 whose bore is, however, exceedingly small. Upon the other hand, muscles 

 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, indeed, contain cavities, but these cavities have not the same 

 significance as those of such organs as, for example, blood vessels whose 

 cavities are essential to the functioning of an organ whose tissues are 

 actively alive. Fully developed bone consists mainly of non-living 

 material and the cavities within it 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. The hollo wness of the brain 

 and spinal cord of the adult vertebrate is an incident of the development 

 of those organs from an embryonic neural tube. The cavities of the 

 brain and cord serve as a channel for a cerebro-spinal fluid which has some 

 metabolic significance, but in the absence of such cavities the metabolic 

 needs of the nervous tissue could doubtless be provided for, as in other 

 massive tissues, by the usual blood and lymph channels. 



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

 medium or to some internal cavity, is a critical region of the organism. 

 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 

 non-living substance which serves as a passive mechanically protective 

 barrier between the living substance and the adjacent space, as when skin 

 produces an external cuticula or a horny layer for protection against 

 external agencies, or as when the lining of a bird's gizzard deposits a tough 



