CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 537 



flow; and we have incidentally mentioned most of the features 

 of oedema, to which it is desirable for us to refer. There is 

 however still one matter to be noted in relation not only to 

 oedema but also to the normal presence of lymph in the lymph- 

 spaces. We remarked in 30 on the peculiar relations of living 

 tissues to water. There are reasons for thinking that the very 

 substance of a cell or a fibre (a muscular fibre for instance) may 

 hold in itself a larger quantity of water at one time than at 

 another ; it may at one time be taking up water from its sur- 

 roundings, and at another time be giving out water to its sur- 

 roundings. Indeed the process of secretion is in large measure 

 based on this action. Now the surroundings from which water is 

 taken up by and to which water is given up to the tissue are 

 furnished by the lymph-spaces. Hence we may suppose that the 

 lymph in the lymph-spaces of a tissue, and so the flow of lymph 

 from the tissue, may, apart from the results of transudation from 

 the blood vessels, be changed in quantity and quality by the 

 action of the living elements of the tissue itself. This may, and 

 probably does, play an important part as regards both the normal 

 flow of lymph and the occurrence of oedema. 



304. Lymph-hearts. In the frog and other amphibia and 

 in reptiles the flow of lymph into the venous system is assisted by 

 rhythmically pulsating 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 pro- 

 cesses 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 orifice 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 

 plexiform 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 mus- 

 cular fibres. Nerve fibres terminate in these muscular fibres, and 

 the muscular wall, unlike that of the blood-heart, is supplied with 

 capillary blood vessels. 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 lym- 

 phatic pores being closed by the approximation of the contracting 

 muscular fibres; when the heart dilates, the fluid in the vein is 

 F. ii. 35 



