COMPARATIVE ANATOMY OF THYROID.—The existence of a thyroid gland in all the Mam malian orders seems to be undoubted ; and though it is probable that, by a close and ex tended survey of the various families, some interesting and perhaps instructive peculiarities might be detected, yet I have not the oppor tunities necessary for undertaking such an inquiry, and can only record one observation where some deviation from the ordinary con dition was discovered. This was in a Rabbit, in which the organ presented the vesicular arrangement much less manifestly than is usually the case. The epithelium consisted of small imperfect celloid particles, disposed so as to form hollow spheres ; but there was scarce any appearance of secretion in the included cavities, which were small, and might at first have escaped observation.
For the following summary of the compa rative anatomy of the thyroid in birds and rep tiles, I am principally indebted to Mr. Simon's paper in the Philosophical Transactions.
In Birds, there are found, in all the va rious orders, two glands, situated one on each side of the trachea, very near the lower larynx, and frequently attached to the jugular veins. They possess the characteristic struc ture of the thyroid body, consisting of a dense aggregation of closed vesicles, which contain a kind of epithelium, and are invested by a close capillary network applied over their homogeneous envelope. The position of these glands with respect to the larynx seems to be neither essential nor constant ; it is however stated by Mr. Simon, that it "always corre sponds to a particular spot of the vascular system, viz., that it lies on the cervical vessels, and receives its supply of blood just opposite to the point at which the vertebral or carotid arteries diverge to their respective destina tions." The following are the details of a mi croscopic examination which I made of these glands in a Pigeon. They consist of closed vesicles about inch diameter, having their homogeneous envelope lined by a rather thick layer of epithelium, so that the cavity is proportionably contracted, sometimes not being more than inch diameter The epithelium consists, for the most part, of nuclei and granular matter. Some of the nuclei are
very perfect, and show a distinct vesicular struc ture, with a well marked peripheral nucleolus. Others are more like granules, solid and opaque, and not above half the size. The nuclei are imbedded in granular matter, which for the most part is diffused freely about, but occa sionally, though very rarely, constitutes the contents of a cell. In some cavities the epi thelium assumes the form of non-nucleated delicate vesicles, of rather large size (fig.742. b.). No secretion, capable of being recognised by the eye, exists in the glandular cavities.
Reptiles.--In the order Chelonia the organ, which is demonstrated by the microscope to be really the thyroid, is found occu pying a definite and uniform position. It lies in the median plane of the body, immediately above the base of the heart, between the two carotid arteries, a ndis overlapped and con cealed by the pericardial lobes of the thymus. The structure of the gland in a young Tor toise I found extremely well marked; the ve sides were large, toAinch diameter, closely aegregated together, and very variously altered from the spherical form by mutual pressure. There was very little investing areolar tissue. The epithelium consisted of a single row of nuclei imbedded in granular matter, which was more abundant than usually is the case. In some parts this had almost disappeared, and was replaced by delicate vesicles larger than the nuclei which lay closely together in contact with each other. Whichever of these forms the epithelium assumed, it did not en croach much on the cavities of the vesicles, which were large and filled with some trans parent non-refracting. fluid.
Most of the cavities also contained one to three yellowish coarsely granular globules, to in. diameter ; these existed in va rious stages of development. A fine large octohedral crystal was also seen in one of the cavities ; but there were no prisms of triple phosphate (fig. 713.).