52 CHEMICAL COMPONENTS OF THE HUMAN BODY. 



either of the histogenetic substances, or of the components of the tissues them- 

 selves ; these substances are all of the saccharine or of the oleaginous class, or 

 are derivable from them by very simple transformations. 



III. The components of the actual living tissues. 



IV. The excrementitious substances, which are formed within the body as the 

 products of the disintegration and retrograde metamorphosis of its tissues, and 

 which are on their way from these to the outlets of the excretory apparatus ; 

 these constituting a group intermediate in their chemical character between the 

 foregoing and inorganic matter. 



It is to be observed, however, that in this classification (as in every other 

 assemblage of natural objects) the different groups are connected together by 

 intermediate links, which render it impossible to isolate them completely. Thus, 

 fatty matters must be looked upon as histogenetic substances, since they seem 

 essential to the production of almost all the tissues, and enter largely into the 

 composition of the adipose and nervous ; yet their production within the body 

 seems to have most immediate reference to the demand set up by the com- 

 bustive process, in which a far larger quantity is daily consumed, than can be 

 required for the maintenance of the organized fabric. So, again, fat as such 

 must be regarded as a component of the actual living tissues, and therefore as 

 belonging to the third class ; but it does not, like the histogenetic substances, 

 undergo organization, being simply combined mechanically with the other com- 

 ponents, and being capable of ready separation from them by no other than 

 mechanical means. On the other hand, although the third class may be regarded 

 as, in great measure, chemically identical with the first, yet the molecular 

 condition of their respective components is very different ; for the properties of 

 the living tissues cannot be chemically examined, until they have been deprived 

 of all those characters which distinguish them as organized substances, and are 

 reduced back to a state resembling that in which they are first taken into the 

 body. Here again, however, we meet with a link of transition; for fibrin, which 

 ranks as one of the histogenetic substances, may be almost regarded as " liquid 

 flesh," ( 24,) and as therefore deserving of a place in the third class. And 

 lastly, the fourth class of substances is connected with the third by this circum- 

 stance, that the excrementitious substances are separated from the blood into 

 which they have been received back, by the agency of glandular structures, of 

 whose organized tissues they form part for a time, as fat does of adipose tissue : 

 whilst, again, they are connected with the second by the fact, that a considera- 

 ble part of the products of disintegration is made use of as calorific material, 

 being converted within the body into saccharine and oleaginous substances in 

 all respects resembling those taken in as food. It will, therefore, be found con- 

 venient to modify the above classification, by arranging the substances which it 

 includes in some degree according to their Chemical characters, still keeping in 

 view, however, their physiological destination. Thus we shall consider in the 

 1st place the substances of the Albuminous type, or " protein-compounds/' both 

 as the materials for, and as the components of, the living tissues; and 2dly, 

 those of the Gelatinous type, under the same aspects. From these we shall 

 pass, Sdly, to the Oleaginous group, of which the chief constituents may be 

 ranked both as histogenetic and as calorific substances, while some members of 

 it should probably rank also as products of disintegration : and in connection 

 with these will be described, 4thly, the Saccharine matters, which are also in 

 part to be regarded as products of disintegration, and which are more exclusively 

 calorific than the oleaginous, being never applied to the formation of tissue save 

 through conversion into fatty compounds. In the 5th place, we shall pass under 

 review the most notable of those metamorphic forms, under which the compo- 

 nents of the tissues present themselves in the principal Excretions. And lastly 



