RESPIRATION, AND AN1MALIZATION. 147 



consequently a single circulation, the course of which, however, is directly 

 the reverse of that pursued in fishes ; for the heart in the present instance 

 propels the blood through the body, and the gills receive it, and propel it to 

 the heart. This is also the case in the snail, slug, and many other soft- 

 bodied worms, which possess a gill in the neck, consisting of a single aperture, 

 which it can open and shut at pleasure. Yet with a singular kind of appa- 

 rent sportiveness, the cuttle-fish is possessed of three distinct hearts, which is 

 one more than is allotted to mankind, in whom this organ is only double. 



In zoophytes we are in great ignorance both as to their sanguineous and 

 respiratory functions. That they stand in need of oxygen, and even of 

 nitrogen, has been sufficiently determined by -Sir H. Davy ; as it has also that 

 they absorb their oxygen and nitrogen, as fishes do, from the water which 

 holds these gases in solution. Their nutrition appears to be effected by an 

 immediate derivation of the nutritive fluid from their interior cavity into the 

 gelatinous substance of their body.* 



Hence then the respiratory organs of the animal kingdom may be divided into 

 three classes; lungs, gills, and holes or stigmata: each of the three classes 

 exhibits a great variety in its form, but the. office in which they are employed 

 is the same. Animals of every kind must be supplied with air, or rather with 

 oxygen, however they may differ in other respects in tenacity of life; for a 

 vacuum, or a medium deprived of oxygen, kills them equally. Snails and 

 slugs corked up in small bottles have been found to live till they had ex- 

 hausted the air of every particle of oxygen, and to die immediately afterward : 

 and frogs and land-turtles, which are well known to survive the loss of the 

 spinal marrow for months, and that of the head or heart for several days, die 

 almost instantly on exposure to a vacuum. f 



Connected with this general subject, there is still an important question to 

 be resolved, and which has greatly occupied the attention of physiologists for 

 the last fifty years. 



Mediately or immediately, almost all animal nutriment, and, consequently, 

 almost all animal organization, is derived from a vegetable source. The 

 blade of grass becomes a muscular fibre, and the root of a yam or a potato a 

 human brain. What, then, is that wonderful process which assimilates sub- 

 stances in themselves so unlike ; that converts the vegetable into an animal 

 form, and endows it with animal powers ? 



Now to be able to reply succinctly to this question, it is necessary first of 

 all to inquire into the chief feature in which animal and vegetable substances 

 agree, and the chief feature in which they differ. 



Animals and vegetables, then, agree in their equal necessity of extracting 

 a certain sweet and saccharine fluid, as the basis of their support, from what- 

 ever substances may for this purpose be applied to their respective organs 

 of digestion. Animal chyle and vegetable sap make a very close approach 

 to each other in their constituent principles as well as in their external ap- 

 pearance. In this respect plants and animals agree. They disagree, inas- 

 much as animal substances possess a very large proportion of azote, with a 

 small comparative proportion of carbon ; while vegetable substances, on the 

 contrary, possess a very large proportion of carbon, with a -small compara- 

 tive proportion of azote. And it is hence obvious, ttiat vegetable matter can 

 .only be assimilated to animal by parting with its excess of carbon, and filling 

 up its deficiency of azote. 



Vegetable substances, then, part first of all with a considerable portion of 

 their excess of carbon in the stomach and intestinal canal, during the process 

 of digestion ; a certain quantity of the carbon detaching a certain quantity of 

 the oxygen existing in these organs, as an elementary part of the air or water 

 they contain, in consequence of its closer. affinity to oxygen, and producing 

 carbonic acid gas ; a fact which has been clearly ascertained by a variety of 

 experiments by M. Jurine of Geneva. A surplus of carbon, however, still 

 enters the animal system through the medium of the lacteals, and continues 



Blumenbach, $ 167. , t See Encyclop. Brit. art. Physiol. p. 679. 



K2 



