CIRCULATION IN MOLLUSCS AND CRUSTACEANS.

101

Among the molluscs, the circulation of the blood in the cuttlefish has a remarkably perfect character. In this animal there are three separate hearts at some distance from each other. Each of these, however, has only one cavity. In short, each of the three is a single ventricle. The cuttle-fish breathes by gills. There are only two kinds of blood, the dark-coloured blood and the red-coloured FIG. 44.

CIRCULATION IN THE CUTTLE-FISH

after Audouin.

e e, lateral or branchial hearts, conveying the blood to the gills g g, whence it is returned to 7, the central or systemic heart, for general distribution.

blood. The organs which contain the dark-coloured blood are the veins of the body, and the two trunks which they form; the two separate hearts, subservient to the circulation respectively of the two gills; the two branchial arteries and their ramifications. The organs which contain the red-coloured blood are the ramifications and the trunks of the two systems of branchial veins, and the systemic heart, or ventricle, in which these two systems terminate; also the aortic system arising from the systemic heart. The middle, or systemic heart, transmits the red-coloured blood by the aorta and its ramifications all over the body; the blood, having become dark-coloured, is carried from the terminations of the aortic system by the veins of the body in two portions to each of the two lateral or pulmonic hearts; from each lateral heart the blood is propelled to the gills of one side, whence, having become red-coloured, it is carried again to the middle systemic ventricle.

Among the Crustaceans, the circulation of the lobster has been particularly studied. In it the heart has a single cavity or ventricle; and from this heart several large arteries are derived, by which the

102 CIRCULATION IN SPIDERS AND INSECTS.

blood is conveyed to all parts of the body; but from one of these arterial trunks branches are given off, which proceed to the gills. The blood is brought back from the several parts of the body by

a, heart; b and c, arteries of eyes and antennæ; d, hepatic artery; e and f. arteries of thorax and abdomen; 9 g, great venous sinus; h h, the gills; i, branchial veins.

FIG. 46.

a

proper veins, and from the gills by branchial veins; and the blood from these two sources mingles in a common cavity, or sinus, before it re-enters the single ventricle, to be again sent forth. Thus in the lobster there are three kinds of blood, - the red-coloured blood, dark-coloured blood, and the blood composed of the dark and red blood mixed together. The red-coloured blood is contained only in the system of the branchial veins; the dark-coloured blood is contained only in the veins of the body; the mixed blood in the venous sinus, or sinuses, where the two kinds mingle before entering the heart. This mixed blood is contained in the heart and in the arterial or aortic system, and also in the branchial arteries sent off from the aortic system.

In the spiders with pulmonary cavitiesthat is, with pulmonary organs limited to one part of the body-there is an elongated dorsal vessel, which gives off arteries and receives the terminations of veins; the action of which seems to be to drive the blood at once to the several parts of the body, and also to the heart; a, trunk passing to pulmonary organ. The purified blood from head; b, vessels commu- the pulmonary organ must mingle with the nicating with respiratory blood returning from the several parts of the

CIRCULATION IN SPIDERS-

after Carpenter.

c, large dorsal vessel, or

organs.

CIRCULATION IN SPIDERS AND INSECTS. 103

body in open spaces, or sinuses, whence, by the branchio-cardiac vessels or veins, it reaches the dorsal heart.

In insects, the circulation of the blood proceeds on a plan altogether peculiar. There is a large dorsal blood-vessel, or heart, provided with apertures and valves, and capable of contraction, but without ramifications, - in short, in insects there is a heart, but no

blood-vessels; and since the air, by the air-tubes extending from the surface, has access to all parts of the body, there is only one kind of blood, namely, the arterial. The dorsal heart extends nearly the whole length of the insect's body; it is open at the anterior extremity, and by this open part the blood issues to diffuse itself over every part of the body. The contractions of this vessel, or heart, begin at the posterior part, and are propagated forwards, so that the contained fluid is pushed from the tail towards the head. Within the vessel are valves, by which it is divided into compartments, so placed that the fluid can pass forwards, but not from before backwards. The several compartments communicate on each side by lateral slits with the cavity of the belly, and these slits are provided with valves, so that fluids can enter from the belly, but cannot again issue from the vessel otherwise than by the opening in front. The nutritive fluid prepared in the intestine percolates through its walls, and mingles with the blood diffused over the body from the anterior openings of the heart; and this mixture of blood and the product of digestion passes into the heart by the lateral openings.

In some of the Radiata a circulation of the blood is admitted;

104

RENOVATION OF BLOOD.

but in this part of Physiology there are too many points of controversy to accommodate it to our limits.

On the Renovation of the Blood by Chyle and the other Products of Digestion. As the blood is unceasingly drawn upon for the repair of the constant waste of the solids and fluids of the body, there is a clear necessity for its continual renovation. Of this reno

vation the most obvious source is the supply afforded of lymph and the products of digestion, chiefly by the thoracic duct, which, as we have seen, communicates with the venous system at the upper part of the chest, on the left side.

The most probable opinion as to the origin of the lymph is, as we have seen, that it is the residue of the liquor sanguinis, returned after nutrition from all parts of the body, to be again mingled with the torrent of the circulation. But whatever its real origin, it must be, in any event, derived wholly from the blood, so that it cannot be set down as an independent source of renovation to that fluid. The chyle, however, does unquestionably contain matter which is independent of the blood, as never having formed any part of that fluid. The chyle is indeed the only distinct organic fluid bearing that character. It cannot, however, be affirmed that the chyle consists wholly of materials derived from without, and that no part of its constitution has been drawn from the blood. It may be regarded, on just grounds, as a general rule of organic nature, that the nourishing matter obtained from without does not become fit to be incorporated with the living solids, until it has united itself with materials prepared within the organism, and derived from the proper substance of the living agent.

We have already traced the chyle to the food which is received into the stomach. In the healthy body that food undergoes a complete transmutation; and until lately it has been universally believed that the chyle and the feculent matter discharged from the lower bowel are the sole products of that transmutation. Doubts have arisen, on

grounds to be stated presently, whether the chyle and the fæces are the sole products of the transmutation of the food in digestion. It is certain, however, that these are at least principal products of that process. However this may be, it is to be remarked that the food is not exclusively the material which undergoes transmutation. The chyle and the faces are the result, whether exclusively or together with other products of the transmutation, of a mass, consisting of the food, mingled with several remarkable organic agents derived from the blood, such as the saliva, the gastric juice, or the proper secretion of the stomach; the bile, the proper secretion of the liver; and the pancreatic liquor, the proper secretion of the sweetbread.

If the food be exclusively divided between the chyle and the fæces, whatever of the mass of food which does not find its way into

SOURCES OF RENOVATION.

105

the feculent discharge, must enter into the constitution of the chyle. Thus, from the character of the feculent mass, some notion may be gained of the relation subsisting between the food and the chyle.

By weight the feculent mass discharged in twenty-four hours. equals nearly one-sixth part of the average daily quantity of food. The solid part of the discharge amounts to about twenty-seven per cent. of the whole, the rest being water, namely, seventy-three per cent. The twenty-seven per cent. of solid matter may be distributed as follows:- Insoluble matters derived from the food, seven per cent.; insoluble matters derived from the bowels, liver, &c., fourteen per cent.; soluble matters, consisting of bile, albumen, extractive, and salts, six per cent.

Thus, in the fæces, the nutritive proximate elements of organic matter, as already referred to, have almost entirely disappeared; there being nothing of that description in that account, except less than one per cent. of albumen, while the average amount of nutritive matter in the substance of a meal can hardly be estimated at less than fifteen per cent. Two important facts here deserve particular remark, the small proportion borne by the faces in weight to the average amount of food, and the minute proportion of nutritive matter which that fraction contains.

Thus, if the chyle and the fæces be the sole products of the transmutation of the alimentary mass in the digestive organs, the chyle must take up nearly all the nutritive matter contained in the food, as well as much of what is not accounted nutritive, together with no small proportion of the matters secreted by the several organs concerned in digestion.

Thus, on the supposition made, if the daily amount of food be estimated at twenty-five ounces, the quantity of chyle which passes daily into the blood must bear a very large proportion to that quantity, and to the nutritive substances, or their products, which that quantity contains.

In estimating the comparative quantities of fæces and chyle, it must not be forgotten that the chyle is more watery, containing about ninety per cent. of water; so that twenty-one ounces of chyle contain no more solid matter than nineteen ounces of fæces. As six or eight ounces of chyle may pass through the thoracic duct in one hour, it is not impossible to believe that from twenty to thirty ounces may pass through that vessel into the blood, in repeated portions, throughout the twenty-four hours. The great quantity of the chyle required to support the common view, hardly tells to its prejudice. But numerous experiments seem to show that true chyle that is, the fluid found in the lacteal vessels and the thoracic duct, at a certain period after food has been taken, and at no other time- does not contain the chief nutritive parts of the food, or their products;