526 OF SECRETION. 



nate either in blind extremities, or in a plexus formed by their inosculation. 

 There are considerable variations in the arrangement of these tubes, however, 

 in different tribes of animals. In Fishes, the Kidneys very commonly extend 

 the whole length of the abdomen ; and they consist of tufts of uniform-sized 

 tubules, which shoot out transversely at intervals from the long ureter. These 

 tubes frequently divide into pairs, but without any great alteration in their 

 diameter. They appear to terminate in coscal extremities, without any inos- 

 culation ; the number of bifurcations, and the degree of convolution, vary 

 greatly in different species. The uriniferous tubes are connected together by 

 a very loose areolar web. The structure of the gland in Reptiles appears to 

 be essentially the same ; its form, however, varies considerably in the different 

 tribes, being greatly prolonged in the Serpents, and abbreviated in the Tor- 

 toises. In the Crocodile, the distinction between the cortical and medullary 

 portion begins to show itself; the tubes being nearly straight where they issue 

 from the ureter, and being convoluted near the surface only of the lobes. The 

 Corpora Malpighiana ( 667), however, where they exist in this class, are 

 scattered through the whole substance ; not being confined, as in higher 

 animals, to the cortical portion. In Birds, the Urinary tubes, forming the 

 several clusters, are more closely united together ; they frequently ramify to 

 a considerable degree. In the Mammalia, as in Man, there is an evident dis- 

 tinction between the straight and the convoluted portions of the system of 

 tubes ; the former character is seen in the medullary substance ; the latter is 

 the cortical. In nearly all below the Mammalia, the kidneys present exter- 

 nally a lobulated aspect ; resulting from the want of union between the dif- 

 ferent bundles of tubes, which arise from separate parts of the ureter. In the 

 kidney of the Mammalia, however, the ureter dilates into a capacious recep- 

 tacle, towards which the several bundles of uriniferous tubes converge, so that 

 they open into it in close proximity with each other; and the lobules formed 

 by these bundles are so closely brought together, that no appearance of a 

 division presents itself, until a section of the gland is made. Among some 

 Mammalia, however, the lower form is still retained ; and it is presented in 

 the Human species also, at an early period of its foetal development. 



667. The following is an account of the structure of the Kidney, according 

 to the most recent investigations. 



a. The distinction between the cortical and medullary parts of the Kidney essentially 

 consists in this, that the former is by far the most vascular, and the plexus formed by 

 the tubuli uriniferi seems to come into the closest relation with that of the sanguiferous 

 capillaries, so that it is probably the seat of the greater part of the process of secretion ; 

 whilst the latter is principally composed of tubes, passing in a straight line from the 

 former towards their point of entrance into the ureter. In this respect there is a consider- 

 able analogy of structure and comparative function, between the two parts of the kidney 

 and the two parts of the brain. The adjoined figure represents the appearances presented 

 by a portion of an injected kidney, as seen by the naked eye, and under a low magnifying 

 power. The tubuli uriniferi, in passing outwards from the calices, increase in number 

 by divarication, to a considerable extent, as shown in Fig. 148; but their diameter re- 

 mains the same. When they arrive in the cortical substance, their previously straight 

 direction is departed from, and they become much convoluted. The closeness of the 

 texture formed by their interlacement with the blood-vessels, renders it difficult to obtain 

 a clear view of their mode of termination. They seem to inosculate with each other, 

 forming a plexus, with a free extremity here and there (Fig. 154) ; each of which (ac- 

 cording to Mr. Bowman) is connected, in the manner to be presently described, with one 

 of the Corpora Malpighiana. The number of these free extremities, however, does not 

 appear to be nearly equal to that of the uriniferous tubes themselves. 



b. Scattered through the plexus formed by the blood-vessels and uriniferous tubes, a 

 number of little dark points may be seen with the naked eye, to which the designation of 

 Corpora Malpighiana has been given, after the name of their discoverer. Each one of 

 these, when examined with a high magnifying power, is found to consist of a mass of 

 minute blood-vessels (Fig. 154,g); somewhat resembling those convoluted masses of Ab- 



