THE FROG 67 



minute pores or stomata perforating the peritoneum. The spaces 

 are well developed in the posterior dorsal region, where they are 

 termed the abdominal or sub-vertebral lymph sacs, indeed so large 

 are they that the kidneys are situate in them. On the surface of 

 the kidney, are minute ciliated funnels leading into small veins, and 

 so putting the lymph sacs in direct communication with the blood 

 stream. The excess of lymph is also returned to the blood by 

 means of two pairs of small pulsating vesicles connected with the 

 lymphatics. The anterior pair are situated immediately ventral 

 to the enlarged transverse process of the third vertebra and open 

 into the sub-scapular vein. The posterior pair lie one on each side 

 of the urostyle and open by a short duct into the ramus com- 

 municans iliacus, a small vein joining the. femoral and sciatic 

 veins. 



The lymphatics plentifully distributed in the wall of the intestine 

 are especially concerned with the collection of the fat obtained from 

 the food, are always full a short time after a meal. The fat is in the 

 form of a very fine emulsion, and so the contents of the vessels have 

 a milky appearance, hence the lymphatics in these parts are often 

 termed lacteals. 



Thus the lymphatic vessels and sacs constitute a part of the 

 circulatory system, and one that is auxiliary to the blood-vascular 

 system. 



Arteries and veins are not only to be distinguished from one 

 another by the fact that the former convey blood to, and the latter 

 from, the heart, but they also differ in structure. 



An examination of a transverse section of an artery shows that 

 its wall consists of three layers or coats. The outer coat or tunica 

 adventitia, consists of a layer of areolar tissue through which pass a 

 number of elastic fibres, and it is for this reason sometimes referred 

 to as the outer elastic coat. The middle coat, the tunica media, 

 is very thick and composed largely of unstriped muscles, most of 

 which run in a circular direction, but some are longitudinal. It also 

 contains a few elastic fibres. The innermost coat, the tunica 

 interna or fenestrated membrane of Henle, is not so thick as either of 

 the others, but is itself composed of three separate layers. The 

 outer is an elastic layer, the middle a layer of fine connective 

 tissue, the sub-endothelial layer, and the innermost is an epi- 

 thelial layer, the endothelium, one cell deep forming the actual 

 lining of the vessel. All these structures together give a tough 

 elastic wall to the artery, which will remain open when cut and 

 empty of blood. 



A transverse section of a vein shows its wall to consist of the 

 same parts as that of an artery, the only difference being the relative 



