Zoology : an elementary text-book / [Sir A.E. Shipley].

  • Arthur Shipley
Date:
1920
Catalogue details

Licence: Public Domain Mark

Credit: Zoology : an elementary text-book / [Sir A.E. Shipley]. Source: Wellcome Collection.

    29/780 (page 5)
    tions. Thus, all the hard skeletons of animals are reallv insoluble * *j excreta. On the other hand, the gastric juice which digests the food in the human stomach, and the slime or mucus, which prevents a frog from drying up when taken out of water, are fluid excreta. A part of the body specially adapted to produce a secre¬ tion is termed a gland. Other products of decomposition reconstitute, as we have seen, the original molecule by combining with the necessary elements from the food; this process is known as anabolism (Gr. avafiaWeiv, to put back or up) or assimilation. Inasmuch as, generally speak¬ ing, from the breaking up of one molecule more than one residue is produced capable of regeneration, there is an increase in the number of biogen molecules causing an increase in bulk of the protoplasm, or growth1. It is believed that both biogen and proteid molecules are of the nature of compound amino-acids. An amino-acid is an acid in which not only is the place of the central acid radicle corresponding to the sulphur in sulphuric acid or the phosphorus in phosphoric acid taken by a group of -atoms containing carbon but this carbon group contains also NH2 the radicle of ammonia. As a consequence an amino-acid can act not only like an acid in combining with an alkali but like an alkali combining with an acid —it is both an acid and a compound ammonia and has in consequence two hands— and owing to this circumstance it can combine with another group similar to itself by uniting so to speak its acid hand with the alkaline hand of the new group or vice-versa and so build up a complex chain. The regeneration of the biogen takes place at the expense of the food. Taking in food is called eating, or ingestion. Since, how¬ ever, the food must penetrate to every portion of the protoplasm it must be dissolved—a process effected by the chemical action of certain products of the decomposition of the biogens, known as ferments. The process is called digestion. The products of digestion must be assimilated; in order that this may be accom¬ plished they are decomposed until quite simple substances are formed ; in a word the amino-acid chain constituted by the ingested proteid is broken into its individual links. The casting out of an insoluble remnant of the food is called defaecation, and inasmuch as such remnants have never formed part of the biogen molecule, 1 See Verworn, General Physiology (Engl. Edition), 1899, p. 486.