104 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



may redissolve again in water. It is not completely coagulated by boil- 

 ing. It is rendered turbid by acetic acid, the cloudiness being removed 

 by an excess of acid or sodium chloride. It is precipitated by nitric 

 acid, potassium ferrocyanide and acetic acid, mercuric chloride, and ace- 

 tate of lead. 



ALBUMINOIDS. 



In the development of the different tissues of the animal body 

 the native albumens already described, which exist in the ovum and • 

 embryonic cells, assume a modified form, the condition under which 

 such a modified albumen is present varying considerably in dif- 

 ferent tissues ; as already pointed out, the difference in the different 

 tissues, especially in the different members of the connective-tissue 

 group, is dependent upon the modification which the albuminous con- 

 stituents of those cells have undergone. Such bodies have a chemical 

 composition very closely allied to that of native albumens. The}' 

 are complex, nitrogenous compounds, but they present certain proper- 

 ties in contrast with the true albuminous bodies. As they appear to 

 result from the transformation of those bodies in the animal economy, 

 not as a rule being found in the vegetable kingdom, they may be spoken 

 of as the albuminoids of special tissues. 



In the connective-tissue group are included a number of different 

 tissues, such as white connective tissue, elastic tissue, tendon, bone, 

 cartilage, and dentine, which at first sight appear to have few if any 

 points in common. Yet all these tissues fulfill the same subservient 

 function of connection and support, all originate from the same layer 

 of the blastoderm, and in different periods of life are often changed 

 from one form into the other. The cells of all these tissues are capable 

 of developing a more or less homogeneous intercellular substance, whose 

 chemical composition differs in the different members of" this group. 



When any of the various forms of connective tissue proper are 

 macerated for some days in lime-water or baryta-water, the various 

 elements fall asunder from solution of the connecting cement, which 

 may be precipitated from its solution by dilute acids. This body is 

 mucin. If the ground substance of the different connective tissues after 

 the removal of mucin is boiled in water, they nearly all yield substances 

 somewhat similar to glue ; hence they are called collagenous bodies. 



We will take these up in turn. 



1. Mucin,— Mucin, or the cement substance, is found in all mucous 

 secretions as a result of special cell action, and in the tissue of mollusks, 

 and is the substance to which their tenacious character is due. It is 

 found in embryonic connective tissue, and serves to bind together the 

 fibres of tendons, and of connective tissue and epidermis, and is found 



