302 Basic Structure of Vertebrates 



function. An organ's primary function usually demands certain acces- 

 sory functions, and a corresponding diversity of tissues enters into the 

 constitution of the organ. In a stomach the primary tissue is the lining 

 layer or digestive epithelium. Muscular, nervous, vascular, and connec- 

 tive tissues play accessory but nevertheless necessary roles. Vascular 

 and connective tissues enter into the constitution of all major organs. 



Anatomy deals with organs as such. Histology concerns itself 

 with the internal and specific structure and organization of tissues. 

 Since the tissue is constituted of cells, histology is necessarily con- 

 cerned with them. Cytology, narrowly defined, deals with cells as 

 such — that is, with that fundamental cell mechanism which is com- 

 mon to all cells and independent of tissue specialization. 



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. The 

 hollowness of the vertebrate brain and spinal cord is an incident of 

 their mode of development and not due to any physiologic necessity 

 which could not be provided for otherwise. Muscles, however, are 

 solid. Connective and skeletal tissues may form bulky solid masses — 

 solid, that is, except in so far 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, ganglions, central nerve-cords, and nerves need not be hollow 

 and ordinarily are not. 



Every surface of the animal, whether apposed directly to the ex- 

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

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

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

 the protoplasm some vitally necessary activity is being carried on — 

 digestion, respiration, absorption, secretion, excretion, diffusion. Or it 

 may be a surface at which the underlying protoplasm deposits a pro- 

 tective nonliving substance. 



Provision for the adequate carrying 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 



