1868.] Prof. W. Jones on the Caudal Heart of the Eel 343 



lie considers to have been through the lymph-spaces op'ened into in the 

 course of the dissection ; and the mode of entrance he considers to have been 

 by suction during diastole of the heart. The sucking action, by which the 

 heart thus draws air or blood into its cavity when the lymph-spaces are cut 

 into, must operate, according to his view, as a means of promoting the 

 flow of lymph in the natural state. After describing the mechanism of 

 the process, the author examines the microscopical characters of the proper 

 m.uscular tissue composing the wall of the lymphatic heart. The result of 

 his observations on this point isj that the muscular tissue of the lymphatic 

 hearts of the frog is similar to that of the veins of the bat's wing, as regards 

 both its granular semitransparent aspect and the breadth of its fibrillations, 

 whilst it differs from the muscular tissue of the blood-heart of the animal 

 in being destitute of transverse markings. 



Part III. ^' Microscopical characters of the rliytlimically con- 

 tractile Muscular Coat of the Caudal Heart of the Eel.^^ Re- 

 ceived April 21, 1868. 



The caudal heart of the eel lies in a kind of framework on the abdominal 

 aspect of the extreme end of the vertebral column. The body of the last 

 caudal vertebra forms the dorsal side of this framework, and a ridge of 

 bone, extending along its concave abdominal aspect, must project into the 

 caudal heart, partially dividing it into right and left compartments. The 

 caudal heart of the eel would thus appear to represent the two caudal 

 sinuses of certain other fishes run into one. 



From the manner in which the caudal heart is connected with the sur- 

 rounding structures of the tail, its movements are communicated to them 

 as described in the author's paper, entitled " The Caudal Heart of the 

 Eel, a Lymphatic Heart," &c. By the elastic recoil of the structures, on 

 the other hand, the cavity of the heart is drawn into a state of dilatation ; 

 and the result must be, as in the analogous case of the anterior lymphatic 

 hearts of the frog, that lymph will be forced into the heart from the 

 adjacent lymphatic vessels or spaces. 



The muscular fibres composing the walls of the caudal heart resemble 

 in shape the sheathed primitive fasciculi of the muscles of the skeleton, 

 but are only half as broad, and they are not transversely striped. They 

 have a granular aspect, and on close examination are found to be a fasciculus 

 of fibrils of an inch broad, contained in a delicate structureless 



sheath. These fibrils resemble the fibrils of the muscular coat of the veins 

 of the bat's wing, and of the muscular coat of the lymphatic hearts of the 

 frog, and may be grouped, the author thinks, together with them under a 

 common head, viz. unstriped rhythmically contractile muscular fibrils. 



