The physiology of the domestic animals; a text-book for veterinary and medical students and practitioners by Smith Robert Meade

Author:Smith, Robert Meade
Language: eng
Format: epub
Tags: Domestic animals, Physiology, Comparative
Publisher: Philadelphia and London, F. A. Davis
Published: 1889-03-25T05:00:00+00:00

SECTION VII. THE CIRCULATION OF THE BLOOD.

IT has been seen that digestion is the preparation of food for absorption ; absorption is the process by which the results of digestion reach the interior of the blood-vessels; but that the blood, which by means of absorption has thus received the nutritive principles of the food, may satisfactorily meet the nutritive wants of the different tissues of the body, it must be in constant motion. The circulation of the blood is, therefore, that function by means of which the nutritive materials supplied by absorption are distributed to the economy after being subjected to aeration, and by which the refuse and effete materials are carried where they may be excreted.

1. GENERAL VIEW OF THE ORGANS OF CIRCULATION. —Circulation is an organic function, being present in both the animal and vegetable kingdoms.

In the simplest forms of life, both animal and vegetable, in which absorption takes place by imbibition from the entire external surface, no special circulatory apparatus is required. It is only when certain tissues become specialized organs for absorption and others for aeration that a necessity arises for some apparatus by which the materials absorbed are convej-ed from the point of absorption to the respiratory organs and to the S3'stem at large. The development of the circulatory-organs is, therefore, proportional to the degree in which absorption and respiration are limited to special tissues.

As might be expected from the definition of the circulation, in the lowest animals, as in plants, in which absorption takes place from the entire external surface, there exists no apparatus for carrying on a circulation of fluid, the contractile vesicles seen in many of the protozoa having, probably, rather a respiratory than a circulatory function; it is only when the digestive organs become highl} : specialized that a circulator}' apparatus appears. Thus, in the coelenterata the somatic cavity is in free communication with the digestive cavit}- and with the exterior, and the fluid which it contains, representing the blood of higher orders, is moved b} r the contractions of the entire body and by the vibration of cilia lining the somatic cavity, there being no indication of either a heart or a vascular system. In the turbellaria, trematoda, and cestoidea the lacunae of the mesoderm and the interstitial fluid of

(491)

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.