CIRCULATION. 631 



It is usually considered that no vessels homologous in character and office 

 with the lymphatics and lacteals of the Yertebrata exist in Non-vertebrate 

 animals ; but it has been suggested that the so-called blood-system of the 

 Mollusca and higher Annulosa, with its usually colorless contents, including 

 corpuscles much more like the white than the red blood-corpuscles of the Ver- 

 tebrata, may possibly be the homologues of the vertebrate lymphatic system. 

 Be this as it may, these vessels are undoubtedly concerned, not only in the 

 function 'of circulation, but also in that of absorption ; for absorbed materials 

 not only pass into the perivisceral cavity, and penetrate the soft tissues of the 

 body, immediately and directly, but they also enter the interior of these so- 

 called bloodvessels, mingle with the circulating fluid, and thus are conveyed 

 to the most distant parts of the frame. Such vessels must be concerned both 

 in the absorption of the food, and in all the phenomena of general extrinsic 

 and intrinsic absorption. In those Non-vertebrate animals, which, as the 

 Annuloida, possess the so-called water vascular system, or some analogous 

 vessels, general absorption may be assisted by them. In the Coelenterata, all 

 of which are destitute of proper vessels, the fine tubular extensions of the 

 body cavity into the soft disc, must aid in this process ; but in the simple 

 hydra, absorption must be accomplished by direct imbibition through the cells 

 lining the digestive cavity, and by general percolation through the soft inter- 

 cellular spaces. In the Protozoa, it must occur through the sarcodous cell- 

 substance, of which those animals consist. 



Whilst, therefore, in the lowest non-vascular animals, nutrient matters at 

 once permeate the tissues which they have to nourish, and whilst, in all ani- 

 mals possessed of vessels, whether absorbent or circulating, or fulfilling both 

 functions, a similar permeation of nutrient matter takes place through the 

 lining membrane of the digestive cavity, yet, in the latter case, it has no im- 

 mediate nutrient action on the solid tissues, but speedily passes into the 

 bloodvessels or absorbents, and thus directly or indirectly enters the circula- 

 ting fluid or blood. Mixed with this, it probably undergoes further elabora- 

 tion, before it again transudes through the walls of the fine vessels, into the 

 solid tissues, which are ultimately nourished by it. 



CIRCULATION. 



We have seen that the absorbent vessels end in the great veins at 

 the root of the neck, and that there, the lymph and chyle are poured 

 into the blood. The blood is not permitted to remain stationary in 

 any part of the living body ; but in order to fulfil its offices in the 

 general functions of nutrition, secretion, and excretion, and its special 

 office of stimulation in regard to the nervous and muscular systems, 

 and in order that it may be constantly purified by the respiratory pro- 

 cess, it is kept in continual motion throughout the whole of life. This 

 motion of the blood takes place, in Man and in most animals, in dis- 

 tinct cavities and channels, viz., through the heart and bloodvessels, 

 the arteries, capillaries, and veins. The movement itself is named, 

 from its definitely recurrent course, the circulation of the blood. 



The general distribution of the arteries, capillaries, and veins of the 

 body, and the structure of these vessels, have already been explained 

 pp. 25 and 54). The heart, or central organ of the circulation, re- 

 quires now to be described. 



