488 



GLAND. 



furnishing about four pints of urine daily, 

 receive, in proportion to their bulk, more blood 

 than the pancreas, where the secerning process 

 is less active. That this is the principle which 

 regulates the supply of blood is also evidenced 

 in the vessels of the mammae, which receive a 

 more ample supply of blood during lactation 

 than at other periods. 



The sangmferous vessels, like the secreting 

 canals, present many varieties in their dis- 

 position in the several glands, the varieties of 

 form in each class being, however, definite in 

 their character, and doubtless having a re- 

 ference to the different kinds of fluids which 

 are required to be separated from the circu- 

 lating blood. Those organs which are pro- 

 vided with a distinct envelope, as the testicle, 

 the kidney, and liver, usually possess but one 

 artery, which enters at the same fissure as the 

 excretory canal ; other glands, presenting a more 

 distinctly lobulated texture and having no 

 proper capsule, the tonsil, pancreas, and mam- 

 ma for instance, receive an indefinite number 

 of arteries, which enter irregularly on all parts 

 of the surface ; lastly, in the mostsimple form, 

 as the mucous crypts, the secreting vessels con- 

 stitute a delicate plexus on the surface of the 

 little bag. 



In all those instances where the gland is 

 large enough to receive one or more arterial 

 trunks, it is found that the vessel having 

 entered begins to divide into smaller branches, 

 which penetrate between the masses of the 

 gland, and these becoming smaller and smaller 

 at length furnish an intricate plexus, the 

 branches of which, as in the case of the simple 

 bag or follicle, ramify on the surface of the 

 blind secreting canals. 



It is only necessary to observe with respect 

 to the veins, that when compared with their 

 arteries, they are smaller than elsewhere ; and 

 also that in common with the veins of the 

 splanchnic cavities, they are devoid of valves, 

 so that in the kidney, liver, &c. they may be 

 beautifully displayed by the aid of a suc- 

 cessful injection, even to their ultimate rami- 

 fications. 



Arrangement of the minute bloodvessels. 

 In considering the intimate texture of the 

 glands, it is essential to state the manner in 

 which the last divisions of the sanguiferous 

 vessels are disposed. By the aid of minute 

 injection these vessels may be demonstrated, 

 though with difficulty, as far as their termina- 

 tion ; and they may also be observed in a few 

 instances during life and whilst carrying on the 

 circulation. 



An opinion to which we shall subsequently 

 recur has been entertained by many anatomists, 

 that the little arteries are either directly con- 

 tinuous with the excretory ducts, or, as we 

 should rather call them, the secreting canals, or, 

 at all events, that some kind of direct commu- 

 nication exists between the terminal arteries 

 and the secreting canals. The most cautious 

 and apparently successful researches, however, 

 do not corroborate this opinion, but, on the 

 contrary, show that no direct communication 

 of any kind exists. In the lungs, which organs 



are formed and developed in exact accordance 

 with the glandular structure, the ultimate divi- 

 sions of the pulmonary artery, after freely 

 ramifying over the surface of the air-cells, are 

 known to terminate by direct continuity in the 

 radicles of the pulmonary veins. Now, that 

 which is demonstrated in the lungs equally 

 applies in the case of the glands. In the 

 simple lacunae of the mucous membrane the 

 arteries are disposed over the surface of the 

 pouch, but they end in the returning veins 

 without opening on the secreting surface. 

 Miiller states that on examining with a suffi- 

 cient power the larva of the triton pulmtris, 

 he observed streams of blood, traversed by 

 single globules, running between the elongated 

 secreting canals of the liver, and, further, that 

 the last arteries pass immediately by a reticu- 

 late anastomosis into the small hepatic veins. 

 This disposition is seen in the adjoining figure, 

 which represents the circuit of the blood in the 

 larva of the triton fifteen lines in length'. 



Fig. 221. 



a, veua cava j b, vena poitanun , c t minute cur- 

 rents of blood in the gall-bladder. 



