318 LYMPH AND LYMPHATIC GLANDS [dl. XXII. 



The muscular coat of these hearts is of variable thickness ; in some cases it can 

 only be discovered by means of the microscope ; but in every case it is composed of 

 striped fibres. The contractions of the hearts are rhythmical, occurring about 

 sixty times in a minute. The pulsations of the cervical pair are not always 

 synchronous with those of the pair in the ischiatic region, and even the correspond- 

 ing sacs of opposite sides are not always synchronous in their action. 



Unlike the contractions of the blood-heart, those of the lymph-heart appear to 

 be directly dependent upon a certain limited portion of the spinal cord. For 

 Volkmann found that so long as the portion of spinal cord corresponding to the 

 third vertebra of the frog was uninjured, the cervical pair of lymphatic hearts 

 continued pulsating after all the rest of the spinal cord and the brain were destroyed ; 

 while destruction of this portion, even though all other parts of the nervous centres 

 were uninjured, instantly arrested the hearts' movements. The posterior, or 

 ischiatic, pair of lymph-hearts were found to be governed, in like manner, by the 

 portion of spinal cord corresponding to the eighth vertebra. Division of the 

 posterior spinal roots did not arrest the movements ; but division of the anterior 

 roots caused them to cease at once. 



Innervation of the Thoracic Duct. By determining the rate of outflow of a 

 fluid at constant pressure passing through the thoracic duct, Camus and Gley have 

 obtained evidence of the presence of nerves, causing both dilatation and constric- 

 tion of the duct. These are contained in the sympathetic chain below the first 

 thoracic ganglion. The effect of stimulation is principally dilatation. 



Relation of Lymph and Blood. 



The volume of bloocl in the body remains remarkably constant. 

 If the amount is increased by injection of fluids, at first its specific 

 gravity is lessened, but in a short time, often in a few minutes, it 

 returns to the normal. The excess of fluid is got rid of in two ways : 

 (1) by the kidneys, which secrete profusely ; and (2) by the tissues, 

 which become more watery in consequence. After the renal arteries 

 are ligatured, and the kidney is consequently thrown out of action, 

 the excess of water passes only into the tissues. 



On the other hand, a deficiency of blood (for instance, after 

 haemorrhage) is soon remedied by a transfer of water from the 

 tissues to the blood through the intermediation of the lymph. 



In severe haemorrhage life has often been saved by injection of 

 saline solution into the vessels, or by transfusion from another 

 person. The transfer of the blood of another animal to the human 

 vascular system is usually dangerous, especially if the blood lias been 

 defibrinated, for the serum of one animal is usually poisonous to 

 another, producing various changes, of which a breakdown of the 

 corpuscles (haemolysis) is the most constant sign. 



Formation of Lymph. 



Carl Ludwig taught that the lymph flow is conditioned by two 



..factors : first, differences in the pressure of the blood in the capillaries 



and of the fluid in the tissue spaces, giving rise to & filtration of fluid 



through the capillary walls; and secondly, chemical differences 



