LYMPHATICS OF VEETEBRATA. 597 



trunk is given off similar to the one that was broken up, it is called 

 bipolar or amphicentric ; if the rete remains broken up, then it is 

 known as a diffuse, unipolar, or monocentric rete mirabile. Some- 

 times arteries only, sometimes veins only (rete mirabile simplex), 

 sometimes both kinds of vessels are united with one another (rete 

 mirabile geminum seu conjugatum) to form the rete. 



Arterial retia are found in the pseudobranchia, in the choroid of 

 the eye of Fishes, and, in very various forms, on the air-bladder. In 

 Birds and Mammals, retia are not unfrequently found in the area of 

 the carotids and their branches. They are very common on the limbs 

 of the Mammalia (Monotremata, Edentata). In the area of the 

 visceral arteries there are both arterial and venous retia ; thus, in 

 the Pig, the mesenteric artery forms an arterial rete. Arterial retia 

 are very common on the terminal branches of the renal arteries, 

 where they form the Malpighian glomeruli, from which, as we all 

 know, another artery is given off to be distributed in the capillaries 

 on the urinary tubules (cf. Fig. 343, B). 



Lymphatic System. 



§ 443. 



The presence of a system of canals connected with the blood- 

 vascular system — in which the nutrient fluid, which has passed out 

 from the capillary portion of the hsemal system, is conveyed again 

 to the blood stream as lymph, after having filtered through the 

 tissues — is an arrangement which is peculiar to the organisation 

 of the Craniota. It appears to be correlated with a high develop- 

 ment of the body, for it is wanting in Amphioxus, and in embryo- 

 logical development it only begins to appear at a relatively late 

 period, and not until the blood-vascular system has been diffeien- 

 tiated into its arterial and venous portion, and is in full function. 

 That portion of the lymphatic system which has its root on the 

 digestive canal is of especial importance, for it receives the chyle, or 

 nutrient material, which has been prepared from the chyme by the 

 process of digestion, and conveys it to the blood-vessels. 



In addition to the function of conveying the lymph, this system 

 of canals has yet another duty, which complicates its anatomical 

 relations. The points at which the form-elements of the lymphatic 

 fluid, or lymph-cells, are developed, are embedded in its vessels ; 

 these lymph-cells are carried to the blood, and are gradually con- 

 verted into its form-elements. 



In the lower divisions of the Vertebrata this lymphatic system 

 has not much independence, for its vessels are chiefly formed of 

 wide spaces, which enclose other organs, and especially arteries. The 

 sheath of connective tissue of the artery also encloses the lymphatic 

 vessel. The veins also may be surrounded by wide lymph-spaces ; 



