3 THE CONNECTIVE TISSUES. [CH. iv. 



But on examining a little more deeply, one finds that the 

 grouping of these apparently different tissues together depends 

 on a number of valid reasons, which may be briefly stated as 

 follows : 



1. They all resemble each other in origin. All are formed 

 from the mesoblast, the middle layer of the embryo. 



2. They resemble each other structurally ; that is to say, the 

 cellular element is at a minimum, and the intercellular 

 material at a maximum. 



3. They resemble each other functionally ; they form the 

 skeleton, and act as binding, supporting, or connecting 

 tissues to the softer and more vital tissues. 



An apology is sometimes made for calling the blood a tissue, 

 because one's preconceived idea of a tissue or texture is that it 

 must be something of a solid nature. But all the tissues contain 

 water. Muscular tissue contains, for instance, at least three- 

 quarters of its weight as water. Blood, after all, is not much 

 more liquid than muscle. Blood, moreover, contains cellular 

 elements analogous to the cells of other tissues, but separated by 

 large quantities of a fluid intercellular material called blood- 

 plasma. 



Blood is also mesoblastic, and thus the two first characteristics 

 of a connective tissue are present. It does not fulfil the third 

 condition by contributing to the support of the body as part of 

 the skeleton, but it does so in another sense, and serves to 

 support the body by conveying nutriment to all parts. 



We may now proceed to a consideration of this long list of 

 tissues, one by one, in the order named. 



Areolar Tissue. 



It is convenient to take this first, as it is a very typical 

 connective tissue. It has a wide distribution, and constitutes 

 the subcutaneous, subserous, and submucous tissues. It forms 

 sheaths (fasciae) for muscles, nerves, blood-vessels, glands, and 

 internal organs, binding them in position and penetrating into 

 their interior, supports and connects their individual parts. 



If one takes a little of the subcutaneous tissue from an animal, 

 and stretches it out on a glass slide, it appears to the naked eye 

 like a soft, fleecy network of fine white fibres, with here and there 

 wider fibres joining it. It is, moreover, elastic. 



But in order to make out its structure accurately it is necessary 

 to examine the thinnest portions of the film with the microscope, and 



