204 THE VASCULAR SYSTKM. 



B. [The Lymph and Chyle. 



The lymph is that portion of the blood which has filtered from 

 the blood-vessels, through the tissues, into the lymphatics. It is a 

 clear, transparent fluid, containing colourless corpuscles, which agree 

 in all particulars with the colourless corpuscles of the blood. During 

 digestion the lymph from the alimentary canal contains particles of 

 fatty matter, and is then known as chyle.] The colourless cor- 

 puscles of the lymph frequently contain portions of red corpuscles 

 or granules of pigment. Non-nucleated masses of protoplasm and 

 decolourised red corpuscles (slightly diminished in size) have also 

 been described as occurring frequently in the lymph (Fuchs 1 ).] 



C. [Histological Remarks on the Vascular System. 



a. The heart has already been described (pp. 216-222). 



b. The blood-vessels present the same structure as the corre- 

 sponding vessels in higher animals. The arteries and veins have 

 each the three walls or layers usually described Tunica adventitia, 

 Tunica media, and Tunica intima. The only characteristic to be men- 

 tioned is that the Tunica adventitia contains branched, pigmented 

 corpuscles. The Nervi vasorum, especially of the smaller arteries, 

 are arranged in two plexuses, one in the Tunica adventitia (His) 2 , 

 the second in the muscular coat (J. Arnold) 3 . The fibres of both 

 plexuses are non-medullated, and have the usual beaded appearance ; 

 they communicate freely with each other. The capillaries present, 

 as a rule, the usual structure : such special arrangements of the 

 capillaries or peculiarities in structure as occur will be described 

 with the organs in which they are found. 



c. The subcutaneous lymph-sacs are lined with a layer of 

 endothelium resembling the endocardium shown in Fig. 140; the 

 endothelial layer possesses stomata (see peritoneum), through which 

 the lymph-sacs communicate with the underlying lymphatics. The 

 endothelium is continued on to the vessels, nerves, etc., which 

 course through the sacs, and excludes these from the cavities. As a 

 rule an artery, vein, and nerve are enclosed in a common sheath. 



d. The lymph-hearts are small saccular bodies with thin trans- 

 parent walls, which contain obscurely striated muscle-fibres. The 

 vessels communicating with the hearts appear to possess valves 



1 Fuchs, Virchow's Arch. 1877, Vol. LXXI, p. 78 seq. 



2 His, Virchow's Arch. 1863, Vol. XXVIII, p. 427. 



3 Arnold, in Strieker's Handbuch d. Gewebelehre, p. 137- 



