EXCRETING ORGANS. 637 



straight converging tubuli meet at their open extremities, 

 and terminate, as shown by Malpighi, in prominent papillae, 

 which are surrounded by calyces opening generally, by in- 

 fundibula, into a common wider receptacle, or pelvis, from 

 which the ureter commences. The tubuli often divide 

 dichotomously, both in the medullary and the cortical part 

 of the kidney, without changing their diameter. The de- 

 velopment of the ureters in the embryo, begins from their 

 renal ends and proceeds downwards, being at first solid, 

 then tubular, then opening into the bladder, which, in some 

 abnormal cases, they do not reach. The urinary bladder is 

 developed from the cloacal end of the intestine, as the pe- 

 duncle of the allantois and the urachus, but its early commu- 

 nication with the alimentary canal is at length entirely cut 

 off in most mammalia, by the separation of the rectal portion 

 of the intestine above from the uro-genital canal below. In 

 the monotrema, however, they continue to communicate, as 

 in reptiles, through the whole of life. 



Many internal secreting glands already considered, may 

 likewise be viewed as partly internal excretory organs. The 

 internal tubuli and cells of the lungs, by the carbonic acid 

 and aqueous part of the blood which they so largely eliminate 

 from the system, may be regarded as presenting an extended 

 internal excretory surface ; and from the composition and 

 functions of the bile, the tubuli of the liver may be viewed 

 nearly in the same light. The various kinds of odorous 

 and poison-glands at either end of the alimentary canal, and 

 even the muciparous glands throughout its entire course, 

 have partly an excretory function. The surface of all mucous 

 membranes lining internal ducts and cavities which communi- 

 cate externally, and serous membranes lining closed cavities, 

 even the interior lining of blood-vessels, by constantly excret- 

 ing and detaching globules, cytoblasts, or scales of epithelium, 

 may also be considered as exerting an excretory function on 

 the circulating fluids of the body. 



