Fig. 445.
To show the circulation of the blood in the frog’s foot, and without causing the animal pain or much inconvenience, it is better to enclose it in a black silk bag, and draw out the foot as shown at a a a, [Fig. 445]. The bag provided should be from three to four inches in length, and two and a half inches broad, shown at b b, having a piece of tape, c c, sewn to each side, about midway between the mouth and the bottom, and the mouth itself capable of being closed by a drawing-in string, d d. Into this bag the frog is placed, and only the leg which is about to be examined kept outside; the string d d must then be drawn sufficiently tight around the small part of the leg to prevent the foot from being pulled into the bag, but not to stop the circulation; three short pieces of thread, f f f, are now passed around the three principal toes; and the bag with the frog must be fastened to the plate a a by means of the tapes c c. When this is accomplished, the threads f f f are passed either through some of the holes in the edge of the plate, three of which are shown at g g g, in order to keep the web open; or, what answers better, in a series of pegs of the shape represented by h, each having a slit, i, extending more than halfway down it; the threads are wound round these two or three times, and then the end is secured by putting it into the slit i. The plate is now ready to be adapted to the stage of the microscope: the square opening over which the foot is secured must be brought over the aperture in the stage through which the light passes from the mirror.
The tadpole circulation is readily seen by placing the creature on its back, when we immediately observe the beating heart, a bulbous-looking cavity, formed of delicate, transparent tissue, through which the blood alternately enters by one orifice and leaves by a more distant exit. The heart, it will be noticed, is enclosed within its pericardium, this being the more delicate part of the creature’s organisation. The binocular microscope should be used for viewing the circulation. Passing along the course of the great blood-vessels to the right and left of the heart, the eye is arrested by a large oval body, of a more complicated structure. This is the inner gill, formed of delicate, transparent tissue, traversed by arteries, and a network of blood-vessels. It is almost unnecessary to say the tadpole has a respiratory and circulatory system resembling those of fishes.
In nearly all fish the heart has but two cavities, an auricle and ventricle; the blood is returned by the veins to the auricle, passes into the ventricle, and is then transmitted to the gills, where, being exposed to the air contained in the water, it becomes deprived of carbonic acid, aerated, and rendered fit to breathe. In the reptile we find a modification of plan. The heart has three cavities, two auricles and one ventricle; by this contrivance there is a perpetual mixture in the heart of the impure carbonized blood which has already circulated through the body, and flows into the ventricle from the right auricle, with the purer aerated blood returned from the lungs, which flows at the same instant into the ventricle from the left auricle.
For the purpose of subsequent observations the tadpole should be selected at a period in which the skin is perfectly transparent, otherwise the appearances already described of the form and situation of the heart, and the three great arterial trunks (proceeding right and left), will not be clearly made out. The anatomical arrangement of the vessels will be seen to be closely connected with the corresponding gill, the upper one (the cephalic) running along the upper edge of the gill, giving off, in its course, a branch which ascends to the mouth, with its accompanying vein; this is termed the labial artery and vein. The cephalic artery continues its course around the gill, until it suddenly curves upwards and backwards, and reaches the upper surface of the head, when it dips down between the eye and the brain.
It must not be supposed that this can be made out in the average tadpole, the obstacle to which is the large coil of intestines, usually distended with dark-coloured food. This must first be reduced by making your tadpole live on plain water for some days. [Plate VII]., No. 158, affords a view of the vessels obtained under the influence of low diet, and whereby we are enabled to trace the course of the three large arteries. The third trunk, traversing the lung, is seen to emerge from the lower edge and descend into the abdomen to form the great abdominal aorta. A small half-starved tadpole shows the heart beating and the blood circulating, but the latter is quite colourless, not a single red globule visible anywhere. The heart is a colourless globe, the gills two transparent ovals, and the intestines a colourless, transparent coil. Through the empty coil the artery is seen on either side leaving the gills, and converging towards the spine, and uniting to form the abdominal aorta, the large central vessel coloured red in the figure. After the aorta has supplied the abdominal viscera, a prolongation, or caudal artery is seen descending to the tail, the all-important organ of locomotion in the tadpole. This artery, entering the root of the tail, is imbedded deeply in the flesh, whence it emerges, and then continues its course, closely accompanied by the vein, to within a short distance of the extremity, where, being reduced to a state of extreme fineness, it terminates in a capillary loop, composed of the end of the artery and the beginning of the vein. The artery, in its course, gives off branches continually to supply the neighbouring tissue. The blood-current in the tail is often seen, even in the main artery or vein, to be sluggish. This occurs independently of the heart, which will continue to beat as usual; it happens, because the circulation in the tail depends very much on the motion of the organ. When this is suspended (as in the confined tadpole under the microscope), the blood moves sluggishly, or stops, till the tail regains its freedom and motion, when the activity of the current is restored.
Having traced the arterial system which conveys the blood from the heart to the extremities, we will now note its return by the veins back again to the heart.
The caudal vein runs near the artery during the greater part of its course, with its stream of blood towards the heart. This stream is swollen by perpetual tributaries from numerous vessels. As the vein approaches the root of the tail it is inclined towards the artery, and diverges from it at the point of entering the abdomen. Here it approaches the kidneys and sends off branches, while the main trunk continues its course onward; and, passing upwards behind a coil of intestine, it approaches the liver, and runs in a curved course along the margin of that organ. The blood is now seen to enter the vena cava by several channels, that converge towards the great vein as it passes in close proximity to the organ. Beyond the liver the vena cava continues its course upwards and inwards to its termination in the sinus venosus or rudimentary auricle of the heart. This termination is the junction of not less than six distinct venous trunks, incessantly pouring their blood into the heart. The circulation in the fringed lips forms a most complicated network of vessels, out of which proceeds a vein corresponding to the artery already traced. This descends in a direct course till it joins the principal vein of the head, which corresponds to the jugular in the mammalia.
Thus it will be seen the blood is driven by the heart into each inner gill through three large blood-vessels, which arise directly from the truncus arteriosus, and may be called the afferent vessels of the gill. In [Plate VII]., No. 156, an enlarged view of a gill is shown.
On closer examination “each internal gill or entire branchial organ is seen to consist of cartilaginous arches, with a piece of additional framework of a triangular form, stretching beyond the arches, composed of semi-transparent, gelatinous-looking material. These form the framework of the organ and support upon their upper surface the three rows of crests with their vascular network, and the main arterial and venous trunks lying parallel to and between them. The three systemic arteries arising, right and left, from the truncus arteriosus, enter each gill on its cardiac side, and then follow the course of the crests, lying in close proximity to them. The upper of these branchial arteries runs alone on the outside of the upper crest, and another branch leaving the trunk and passing into the network of the crest, whence a returning vessel may be traced carrying back the blood across the branchial artery, and to a vessel lying close to and taking the same course as the artery itself. Carrying the eye along the latter vessel we find, at a short distance from the first of these crest branches, a second, leaving the main trunk and entering the crest, when a corresponding returning vessel conveys the blood across the arterial trunk into the vessel lying beside it, as in the former instance. A number of these branches may be traced from one crest to the other. But it is now seen that the trunk from which these arterial branches spring diminishes in size as it proceeds in its course (like the gill artery in fishes), while the vessel running parallel to it and receiving the stream as it returns from the crest enlarges to some extent. Thus, the artery or afferent vessel which brings the blood to the gill is large at its entrance, but gradually diminishes and dwindles to a point at the opposite end of the crest; while the venous or efferent vessel, beginning as a mere radical, gradually enlarges, and thus becomes the trunk that conveys the blood out of the gill to its ultimate destination. This vessel is the upper branchial vein so long as it remains in contact with the gill; subsequently it changes its name on leaving the gill and as it passes upwards for distribution to the head, when it is designated the cephalic artery. The middle branchial artery and vein proceed in like manner in connection with the middle crest, and the lower artery and vein in connection with the lower crest. The middle and lower venous trunks, having reached the extremity of the crests, curve downwards and inwards, and leave the gill. The former trunk, converging towards the spine, meets its fellow, and with it forms the ventral aorta. The latter gives origin to the pulmonary artery, and supplies also the integuments of the neck. Curious and interesting is the final stage of the metamorphosis, when the waning tadpole and incipient frog coexist, and are actually seen together in the same subject. The dwindling gills and the shrinking tail—the last remnants of the tadpole form—are yet seen, in company with the coloured, spotted skin, the newly formed and slender legs, the flat head, the wide and toothless mouth, and the crouching attitude of the all but perfect reptile.”[87]