124 
DR. C. H. JONES ON THE STRUCTURE 
These and several of the observations previously recorded render it, I think, certain 
that the formation of perfect cells is not necessary to the act of secretion, the nuclei 
alone are, doubtless, adequate to produce the necessary change in the exuded plasma ; 
but if it be intended that the structure shall have a certain degree of permanence, 
then it is surrounded with an envelope, to isolate and confine in some measure the 
elaborated contents. 
In the Pigeon, the ducts, when dissected out, were found to terminate very much 
in the same manner as they do in the liver of mammals ; thus one small branch, r^g^tli 
of an inch in diameter, appeared to terminate by a closed extremity ; its walls consisted 
of nuclei set close together in an amorphous or finely granular basis-substance, and 
were not invested by any basement membrane ; another branch, j-^^th of an inch 
in diameter, was seen in one part of its course to have a fine homogeneous tunic 
enclosing nuclei and granular matter ; it terminated by losing its tubular character, 
and becoming resolved into a tract of very perfect and beautiful nuclei, which still 
retained the original form of the duct. In an owl I found one or two minute ducts 
which certainly seemed to terminate by closed extremities ; they consisted principally 
of a finely granular substance containing small greenish yellow oil-drops ; nuclei were 
seldom visible in them, and the basement membrane ceased before their terminal 
extremity. In an examination of a duck I obtained some very perfect specimens of 
the ultimate ducts ; they were often long and slender, about y^^th of an inch in 
diameter, and tapered very gradually to their extremity, which I think was in some 
instances certainly closed. In structure they resembled pretty nearly those of the 
Owl, appearing as cylindrical tracts of granular matter, but the nuclei in them were 
rather more distinct ; in one of them which I have figured it is well seen how the 
basement tissue, distinct at one part, gradually fades away, and is lost as the duct 
diminishes toward its terminal extremity. From these observations there can be no 
doubt that the liver in birds is of the same type of structure as that which we have 
found to prevail in the two lower Vertebrate classes; we may remark again, in these 
animals of rapid circulation and active respiration, how the character of the oxidizing 
process affects the condition of the liver, a very small quantity of oily matter only 
existing in the parenchyma of the Pigeon, a bird of vigorous flight, while that of the 
stately and slow-moving Swan presents evident traces of retained secretion. 
Lastly, we arrive at the Mammalia, in whom each of the glandular organs has a 
definite and unvarying type of structural arrangement. The tubes of the testis and 
kidney, the vesicles of the salivary and allied glands, are familiar to our thoughts 
and observations, but the well-defined and closely-crowded cells of the liver are yet 
scarcely acknowledged to constitute a parenchyma in the true acceptation of the 
word, several yet seeming to incline to the belief that they are contained in some way 
in terminal expansions of the ducts. In endeavouring to determine the long-mooted 
question, as to the mode in which the biliary ducts terminate, I have of course re- 
sorted to the method of injection, but except in the case of the Pig, I cannot think 
