Comparative structure. — I am not aware of any observations upon the comparative structure of veins in birds and reptiles. Those that I have myself made are principally in troduced under the heads, Fenestrated mem brane, Epithelium, 6-c.
In Birds, the venous walls are composed principally of areolar tissue, external to the lining membrane; and I have observed that in the diving-birds, the vena eava posterior is composed almost solely of elastic tissue. There are no muscle-cells in the cerebral veins of birds.
In Fishes, the venous parietes appear to be formed of a sort of areolar tissue longitudi nally disposed. Embedded in the walls of the veins there is an abundance of pigment, which exists in greater quantity in the inter costal veins, and those tributary to the car dinal vein. This pigment is of the variety called by Schwann " stellate pigment cells," and consists of branched cells, of a black colour, and various form. The body of the cell is generally some modification of an oval or oblong, and lies with its long axis corres ponding to that of the vessel ; and from its extremities caudze project, of every imaginable shape and proportion, of larger size than those which project from its sides, which are few and small. The body of each cell appears to be occupied by its nucleus.
ventricle, by whose contractions the blood is propelled in the vein at the commencement of which it is placed.
Rusconi has differently interpreted this organ, and calls it a lymphatic heart, believing that it is perfectly analogous to those pul sating sacs, which are found in certain parts of the lymphatic systems of various reptiles. And certainly the occurrence of a little pul sating organ, having the form, colour, and general aspect of the lymphatic hearts in the frog, situated in the vicinity of the venous system, having an action independent of the systemic heart, and existing in a cold-blooded animal, is, from analogy, enough to suggest the idea that Rusconi's view is the correct one. But this is not confirmed by anatomy, for after repeated and numerous observations of this heart, I am convinced that the original description of it, given by Dr. Hall, is the truth.
The caudal heart is to be found in eels of all sizes, but is best seen in small individuals a few inches in length, on account of their clear skin, and the larger proportional size of the organ in them. It may be seen by holding the eel's tail between the light and the eye of the observer, either with a lens, or without the aid of any magnifying power. It may be con veniently examined with the microscope by wrapping the head of the animal in a wet cloth, and then applying its tail to a slip of glass, and placing it under the field of the instrument ; but the extreme restlessness of the little fish some times renders it necessary that it should be partially or completely stunned before it can be viewed. The heart is then seen to be
In the accompanying figure, some of these cells are represented from the eel (Anguilla acutirostris), and sole (Solea vulgaris), which present considerable variety of form. They also vary in the same individual in different situations : in the cardinal vein they are most numerous and aggregated ; in smaller veins they are less numerous and more distinct ; and in those of minute size, they only occur here and there at considerable intervals, and at the spots where they are placed they occupy nearly the whole face of the vessel.
When submitted to high microscopical power, these cells are seen to be mixed up with the tissue of the venous coats, and their thin branched extremities have, in some places, much the aspect of elastic tissue, in the uni formity of their dimensions and their dicho tomous divisions.
Caudal venous heart of eel. —Dr. Marshall Hall, in 1831, first figured and described a peculiar pulsating organ which exists in the tail of the eel, calling it by the name of "caudal heart ;" and he considered it to be a muscular placed near the extremity of the cardinal vein, on the hmmal aspect of the caudal vertebra; at the end of the tail. It is of a yellowish colour, chequered more or less with stellate pigment, and of a form varying from a pear shape to a spindle-shape. (Fig. 868.) At the distal extremity it is connected with a small vein, which collects the blood from the capillaries of the tail ; at its proximal ex tremity it is connected with the commenc ing cardinal vein ; or it may be said that on either side is the cardinal vein, and' hat the heart is a muscular development on the coats of that vessel. The action of this heart ap pears to be quite independent of the branchial heart ; for, as Dr. Hall observes, " whilst the latter beats sixty, the former beats one hun dred and sixty times in a minute. It con tinues for a very long time after the influence of the pulmonic heart is entirely removed."* I have never seen the action of the caudal heart quite so rapid as is described by Dr. Hall : in one individual, partly torpid from cold, it contracted but sixty times in• the minute ; but when this fish was warmed, the number rose to one hundred and two, — the beats of the branchial heart being less than half the caudal.