Not only did Ituysch adopt a most in sufficient mode in prosecuting his inquiries, but he assumed as a fact what was in reality a mere hypothesis, that secretion can only take place from the open mouths or orifices of the secerning arteries. The only point, therefore, which he discussed was, whether the passage of the arteries into the excretory ducts takes place gradually and insensibly, or suddenly and by the intervention of a follicle ; for it never occurred to the anatomists of those times, or even to Haller and his contemporaries, that canals closed at their end by cul-de-sac, and without open arterial mouths, could secrete.§ But the true opinions of Malpighi did not refer to the exact mode of termination pos sessed by the arteries; nor did he imagine that any particular machine or follicle was interposed between the arteries and the ducts : his observations were rather directed to the more important circumstances relative to the disposition, formation, and extent of the true secreting canals.
In concluding these remarks on the hypo thesis of :Malpighi, it is due to the character of that illustrious cultivator of anatomical sci ence to state that his views are highly phi losophic, and in a general manner correct— that they are supported by numerous obser vations made on the glands of the lower ani mals, as well as on the development of the liver during incubation—and that he had thus the sagacity to adopt the mode, which expe rience has shown is alone capable of resolving this difficult question.
It would be superfluous to enter into a de tailed account of the opinions advanced by later anatomists, as they are for the most part simply modifications of the hypothesis either of Malpighi or of Ituysch. A few general observations will therefore suffice.
Ferrein has the merit of being the first wri ter who pointed out in a more distinct manner than had been done by 'Malpighi, the great importance of what are erroneously called the excretory ducts, but which constitute, as we have already shown, the true secerning struc ture. Ile remarks" that the cortical Fart of the kidney is composed of a collection of white cylindrical tubes, variously folded on them selves ((armies eorticales, or ducts of Ferrein,) and he thought he had seen the same tubes in the liver. The serpentine cortical canals have been seen in birds by Galvani, to be filled with cretaceous urine after the ligature of the ureter. Although the researches of Ferrein are very important, yet they want that support from comparative anatomy, by which means alone they could have been made subservient to esta blish any general principles.
To Rolando belongs the honour of having demonstrated the mode in which the glands are developed from the alimentary canal. fly
carefully conducted observations on the in cubated egg, he discovered that each of these organs in the first instance consists of an ele vation or tubercle of the intestine, which sub sequently becomes hollowed and forms a atrial directly continuous with that of the intestine. He also distinctly announced what has since been demonstrated in all its details, that the lungs are formed, like the glands, by a pushing ma of the upper end of the intestinal tube ; and he further describes the mode in which the bronchi and their subdivisions are deve loped. The error of those writers who contend with Ruysch that the bloodvessels and the secreting canals are continuous with each other, is clearly shown ; in short, Rolando was the first modern anatomist, who, following in the footsteps of Malpighi, pointed out the manner in which the inquiry ought to be prosecuted, and thus laid the foundation of those laborious and interesting researches for which science is principally indebted to the German anatomists, and by which, within the last few years, the subject of the glandular organization has been so strikingly elucidated.* Development.— The investigations of Ilar vey, Malpighi, Rolando, Weber, Meckel, Biir, Valentin, Ratlike, Muller, and many other anatomists, have very satisfactorily determined the manner in which the glands are in general developed. It is, however, necessary to pre mise that these Observations principally relate to those glandular organs which are appended to the alimentary canal, especially the salivary glands, the pancreas, and liver ; for as regards the development of those glands that are subordinate to the secretion of urine and to generation, comprising essentially the kidney and the testis, much uncertainty still prevails, although it is rather generally believed that the corpora Wolffiana, or false kidneys, are in some way or other connected with their primary for mation:I From the researches that have been made with so much care, we learn that, although there are many modifications of the formative process in the different classes of the glandular organs, there are yet certain fixed laws in obedience to which they are produced. As the development of the individual glands is, how ever, considered in the several articles on those organs, it is only requisite to describe in this place in a general manner, and without no ticing the modifications of the general rule, the process of formation. In prosecuting this inquiry two different objects present themselves for examination,—the primitive substance in which the gland is developed, and the internal component parts, consisting essentially of the secreting tubuli and the bloodvessels.