848 



Vertehrat I 



Peculiar to the Vertebrata is tlie so-called lympliatic system, 

 a special system of canals and spaces, distributed over the whole of 

 the body, and containing fluid. Its function is in part to reabsorb 

 the plasma which has escaped from the capillaries into the tissues, 

 in part to take up fluid nutritive substance (chyle) from the 

 wall of the alimentary canal, and to carry both into the blood. 

 Its principal trunks open into certain large veins. In the lower 

 Vertebrata (Pisces, Amphibia, Reptilia), the .lymph vessels occur 

 partly as sheaths round the arteries and veins, whilst they are 

 elsewhere represented by special vessels which are, however, to some 

 extent irregular in form, sometimes wide or saccular. There are 

 often large lymph sinuses, e.g., below the skin in the Frog. Usually 

 near the point of entrance into the veins, the large lymph vessels 

 are rhythmically contractile, the lymph hearts; in the Frog, for 

 instance, there is a pair posteriorly on the dorsal surface ; they are 

 absent from the Mammalia, but present in all other classes. The 

 fluid in the lymphatics is colourless or whitish, and contains numerous 

 leucocytes, identical with the white blood corpuscles. They are 

 formed in very cellular portions of the connective tissue attached 

 to the lymphatics, and, breaking loose, are carried away by the 

 lymph ; this tissue frequently forms specialised rounded bodies, the 

 so-called lymph follicles, which, especially in the Mammalia, 

 are collected into large lymphatic glands. An organ in 

 connection with the true vascular system, which serves also for the 

 production of white blood corpuscles, is the spleen, a dark-red 

 body of considerable size, situated in the abdomen near the stomach. 

 The red blood corpuscles are formed chiefly in tlie spleen and in 

 marrow. The epithelial lining of some of the blood 

 vessels in these parts is much thickened and stratified ; 

 cells of this epithelium develop into blood corpuscles, 

 break ofl: and enter the vascular current. 



The kidneys, which lie on the dorsal 

 wall of the body-cavity,consist of innumerable 

 long, coiled glandular tubules, the urinary 

 tubules, bound together by connective 

 tissue. The closed end of the tubule, which 

 is somewhat expanded, is invaginated to 

 form a capsule (Bowman's capsule), 

 by the inpushing of a small rete mirabile, the 

 glomerulus. This results from the break- 

 ing up of a small artery, its capillaries again 

 uniting into a single artery, which, later, 

 goes to form the capillary network of the 

 kidney. The thin walls of the glomerulus 

 appear to excrete the watery part of the 

 urine, osmotically, whilst the salts are sepa- 

 rated by the kidney tubules. In many Fish 



Fig. 287. Diagram of 

 the end of a urinary 

 tubule of a Vertebrate. 

 u urinary tubule, which 

 splits into two branches, of 

 which one ends in a ciliated 

 funnel, /, the other in a 

 Bowman's capsule, b. a 

 afferent, e efferent arteries. 

 — Orig. 



