24 



COMPARATIVE ANATOMY. 



tissues present this common character in tlieir more intimate structure. 

 They are called connective substances, as the majority of these 

 tissues serve to unite other tissues to organs, or systems of organs. 



The differences in these tissues are due partly to the character 

 of their cells, partly to their relations with the intercellular sub- 

 stance, and partly to the chemico-physical constitution of the 

 intercellular substance; but all these poiuts are not equally well 

 marked in every part of them. Whilst this latter circumstance 

 allows us to recognise the passage of one of these tissues into the 

 others, the fact that such passages do periodically take place under 

 our observation, affords a more weighty reason for uniting them 

 than the fact that we can detect common characters in their 

 structure, although often hidden by various differences. The 

 various tissues which belong to this group are : 1) cellular con- 

 nective tissue, 2) gelatinous tissue, 3) fibrous connective tissue, 

 4) cartilaginous tissue, 5) osseous tissue. 



§ 21. 



Connective tissue is divided into the following varieties: 

 1) Cellular connective tissue (vesicular connective tissue) is 



the simplest form. It is formed 

 of rounded or elongated cells, 

 which are separated by a small 

 quantity of intercellular sub- 

 stance only. There are often 

 vacuolated spaces in the cells, 

 which are filled with a fluid. The 

 intercellular substance often has 

 the form of cell membranes, which 

 serve to unite the juxtaposed 

 cells to one another, and are 

 common to neighbouring cells. 

 In other cases again it is more 

 largely present, without prepon- 

 derating in quantity over the 

 cells. The differentiation of the 

 protoplasm of the intercellular 

 substance varies in degree. This 

 tissue is most widely found in 

 the Ai'thropoda and MoUusca. 

 In the Vertebrata it forms the 

 chorda dorsalis, or notochord. 

 2) Gelatinous tissue 



Fig. 8. From the gelatinous substance 

 of the disc of Aurelia aurita, treated 

 with iodised serum (after M. Schultze). 



X 500. a Branched fibres in which no (mucouS tissue) is distinguished 

 cells can be made out. h Cells in the i^y the Soft gelatinous character 



homogeneous gelatinous substance : the p .i • j_ n i i j_ 



Drocesses are largely retracted in this ^^ ^^^ mtercellular Substance; 



processes are largely retracted in this 

 specimen. 



it is ordinarily hyaline, and in it 



