324 THE TISSUES AND MECHANISMS OF DIGESTION. 



But these are pathological questions into which we must not enter here. 

 We have touched upon them because they illustrate the important processes 

 taking place in the lymph-spaces, and, as we have more than once insisted, 

 the lymph in the lymph-spaces is the middleman of all the tissues, and hence 

 facts illustrating the laws which govern the flow of lymph into and out of 

 the lymph-spaces are of fundamental physiological importance. 



257. Lymph-hearts. In the frog and other amphibia and in reptiles 

 the flow of lymph into the venous system is assisted by rhythmically pul- 

 sating muscular lymph-hearts, which present many curious analogies with 

 the blood-heart. The frog possesses four lymph-hearts. Of these, two 

 belonging to the hind limbs are placed one on each side of the coccyx near 

 its end, and, being covered only by aponeurosis and the skin, may without 

 dissection be seen beating. Two anterior ones are placed on the transverse 

 processes of the third vertebra, and are covered from view by the shoulder 

 girdle. Each lymph-heart is a more or less oval sac lying in one of those 

 lymph sacs or cavities lined with sinuous epithelioid plates, which, as we 

 have said, are present in the frog. It is continued at one end, by an ori- 

 fice guarded with valves, into a small vein which opens, in the case of the 

 posterior heart, into a crural vein, and in the case of the anterior hearts into 

 a jugular vein. The wall consists of muscular fibres arranged in a plexi- 

 form manner and supported by a considerable amount of connective tissue. 

 These fibres are striated and* branched and are intermediate in character 

 between cardiac and skeletal muscular fibres. Nerve-fibres terminate in 

 these muscular fibres, and the muscular wall, unlike that of the blood-heart, 

 is supplied with capillary bloodvessels. The interior is lined with epithelioid 

 plates of sinuous outline, and this lymphatic lining is continued along a 

 number of openings or pores, by which the cavity of the heart opens into 

 the surrounding lymph-space. When the heart contracts the contents are 

 driven into the vein, the lymphatic pores being closed by the approximation 

 of the contracting muscular fibres ; when the heart dilates the fluid in the 

 vein is prevented from returning by the valves at its mouth, while the lymph 

 enters readily from the surrounding space through the now open pores. In 

 the frog regular lymphatic vessels are scanty ; hence these lymph-hearts 

 become of considerable importance in promoting the flow of lymph. The 

 lymph-hearts of reptilia are similar in structure and function. In the frog, 

 in which they have been chiefly studied, the action of the lymph-hearts is in 

 a measure dependent on the spinal cord. The posterior lymph-hearts belong- 

 ing to the hind limbs are connected by means of the delicate tenth pair of 

 spinal nerves with a region of the cord opposite the sixth or seventh vertebra 

 in such a way that section of the nerve or destruction of the particular region 

 of the cord suspends o** destroys their activity. The anterior pair are simi- 

 larly connected with a region of the spinal cord opposite the third vertebra. 

 Each pair therefore seems to have a " centre " in the spinal cord ; but it is 

 probable, though observers are not wholly agreed, that the hearts, after 

 destruction of their spinal centre, ultimately resume their rhythmic beats, 

 so that the dependence of their activity on the spinal centre is not an abso- 

 lute one. Like the heart of the blood -system, the lymph-hearts may be in- 

 hibited, and that in a reflex manner, the inhibition centre being moreover 

 in the medulla oblongata. If a frog be carefully observed, the activity of 

 the lymph-hearts will be found to vary largely, and these variations appear 

 to be in part due to nervous influences, so that in this way the movement 

 of lymph, and hence the processes of absorption, are in this animal directly 

 dependent on the nervous system. 



