CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 509 



Still another kind of oedema is one due to changes taking 

 place in the blood, quite apart from variations of blood-pressure. 

 This kind of oedema is seen in some diseases of the kidney, in 

 " Bright's disease " for instance. In such cases the blood contains 

 less proteids, and indeed less solids, is more watery and of lower 

 specific gravity than is normal. But the oedema is not in these 

 cases to be explained on the view that the more watery blood 

 passes more readily through the capillary walls, for it maybe shewn 

 experimentally that the mere thinning of the blood, as by the 

 injection of normal saline solution into the blood vessels, will not at 

 once lead to oedema, at least in the limbs and trunk, and it is these 

 which in Bright's disease especially become oedematous. In all 

 probability the oedema of Bright's disease if it be really due to the 

 abnormal character of the blood, is produced by the abnormal 

 blood so acting on the blood vessels that these allow a transu- 

 dation greater than the normal. 



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. 



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 vertebrae, 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 



