§ 351. Thyroid of Mammalia.-The representative beginnings of the vasoganglion commonly known as the thyroid gland are noticed in vol. i. p. 564 (Fishes, Reptiles), and in vol. ii. p. 230 (Birds): but this organ is recognisable, without ambiguity, only in the present class. Here it is locally related to the windpipe, and has received its name from its proximity to the shield-shaped cartilage of the larynx in the human subject. It consists of a pair of oblong, rounded masses; in some, especially higher gyrencephalous Mammals, united as in Man by a transverse band of like substance crossing the sternal aspect of the air-tube.

The proper tunic of the thyroid is a thin layer of condensed areolar tissue, from the inner surface of which proceed septal or trabecular processes, partitioning its substance into lobules, and ultimately into minute bags of vesicles. The analogy to the structure of the spleen is close, but the frame-work is much less dense and fibrous and the vesicular structure, instead of receiving the blood directly, is filled with a solution of fibro-albuminoid, proteine, or myeline substance derived therefrom. The quantity of blood sent to the thyroid is much more than would be needed for mere nutrition: it is derived from arteries, not constantly rising just beyond the points where the arteries to the brain are given off from the large trunks, but varying according as the length of the neck in Mammals may affect the relative position of the thyroid to those trunks: thus in the Giraffe and most Ungulates the arteries supplying the thyroids come off from the contiguous part of the carotids. There may be two or three branches from the common carotid (Lutra); and the distinction between lower thyroid arteries' from the subclavian, and upper thyroid arteries' from the ectocarotid, hardly begins to be established before the Quadrumanous order is reached. The ultimate ramifications of these arteries form close-meshed plexuses upon the limitary membrane or capsule of the vesicles; such capillaries present a diameter of from th to 30th of an inch. The blood is returned by veins joining in most Mammals the external jugular; and in Quadrumana and Man the internal jugular: but with varieties in this respect.

2000

1

The effect of 'formifaction,' or assumption of shape and definable size, by the colloid,' 'proteine,' or 'myeline' elements of the solution filling the thyroid vesicles, is shown in the sections of such from the Hedgehog, fig. 435, and the Bullock, fig. 437, and in portions of such lining, or adherent formed matter, from the thyroid vesicles of a Rabbit, fig. 436. In these instances

the forms have been described as an epithelial stratum, consisting usually of nuclei set closely together in a scanty basis substance, fig. 435, which is either feebly granular or of a somewhat oily aspect:' their nucleoli are not always visible, The nuclei are and vary in number from one to four or five. always vesicular, bounded by a strongly marked envelope, and have a mean diameter of 5th inch.'1 But the formed lining

substance often presents, as in figs. 435 and 437, the condition of delicate vesicles, without nucleus, with contents mostly pellucid, sometimes faintly granular. Dr. Jones observes:-'I am inclined to believe that they originate in the nuclei, which undergo a kind of expansion, at the same time losing their nucleoli.' Emancipating himself for a moment from the 'generative' theory in reference to the progress of the nucleus from its primitive condition to a further stage of cell-development,' he candidly admits it to be worth remarking that it' (the stage) may be artificially produced by adding to the specimen some coagulating reagent, which speedily solidifies a film of albuminous plasma around the nuclei, and thus produces very good imitations of cells.''

6

Analyses of the contents of the thyroid have shown or rendered it very probable that they are albuminoid, yet not in the state of ordinary fluid albumen, and that gelatine is sometimes an ingredient: among the salts are chloride of sodium and a trace of alkaline sulphate: crystals of triple phosphate and of oxalate of lime occur in the cavities.3

In the Ornithorhynchus two bodies, extending between the

1 CCLXXIX. p. 1104.

2 Ib. p. 1105. For the conditions and degree in which this and most other phenonomena of so-called cell-development' may be artificially manifested, see CCIx" and ccx", especially the latter important contribution to the philosophy of physiology.

3 CCLXIX. p. 1106.

scapula and humerus, covered by the panniculus carnosus and the trapezius, present a reddish colour, a lobulated structure, and pretty firm texture, and seem to represent the thyroids. These are in more constant relation to the windpipe, in Marsupials: they are two disunited bodies in the Dasyures; each presenting the size of a horse-bean in the Das. macrurus. They were of the same size in a Phalangista fuliginosa, but were united by a filamentary strip passing between their lower extremity, across the first tracheal ring. In the Wombat I found two elongated thyroid bodies of a dark colour reaching from the thyroid cartilage to the seventh tracheal ring on each side. In the Koala they were situated lower down, extending from the fourth to the ninth or tenth tracheal ring.

The thyroid is relatively small in the Kangaroo. It presents more normal proportions in Rodents, but is connected by very lax areolar tissue to the trachea. Each body is elongate and almost cylindrical, but expanding at the lower end, where they are joined by a thin band, in the Hare and Rabbit. The uniting band is thicker and rounded in Rats and Marmots; but appears to be wanting in Geomys and Bathyergus. The thyroid bodies are commonly ununited in Cheiroptera. They lie, similarly detached, but low down, opposite the sixth and seventh tracheal rings, in the Elephant. They are also separate and more remote from the larynx in Delphinidæ. Cuvier notes them as rounded and separate in the Hyrax. In the Rhinoceros I found them joined together by a very thin and narrow strip continued between their hinder ends, obliquely across the trachea. Each body was elongate, subtriangular, extending from the sides of the larynx to the fourth tracheal ring, and diminishing as they descended: a small compact yellow body was attached to the thyroid at the point of emergence of the vein. In the Horse, also, I find the thyroids connected by a slender band crossing the second tracheal ring: each body is egg-shaped and united about one-third from the lower end. The thyroids are relatively smaller in the Ass, but are similarly united to each other.

In the Llamas (Auchenia) the thyroids are oval, with the great end downward, extending from the side of the thyroid cartilage to the third tracheal ring, where they are connected together by a filamentary band: this band is relatively broader in the true Ruminants, in most of which the thyroids have a more

I regret that I omitted to note the condition of the thyroid in CLIII".

2 Cuvier describes them as 'entièrement séparés, et situés bien au-dessous du larynx. XII. tom. viii. p. 677.

elongate form. In Bears the thyroids are joined by a long slender band at their lower ends. In Felines the uniting band appears to become longer and more slender by age, and sometimes disappears. Cuvier notes three distinct connecting bands in a Civetcat,' and two such bands in a Marmoset monkey. In the Ayeaye the thyroid bodies, elongate, triangular, and flattened, lic upon the sides of the second to the seventh tracheal rings inclusive, and are devoid of connecting transverse strip. In most Quadrumana the thyroids are united, but by a longer and narrower band or isthmus' than in Man. In him the thyroid bodies are not only relatively large, but are united by an 'isthmus' so broad

a

438

as to usually extend across

two or more upper rings of the
trachea; moreover, a process
extends from the upper part
of the isthmus, as the 'py-
ramid or mesial column,'
which in some subjects reaches
to the hyoid bone. Many
varieties have been noted in
the human thyroid. Some-
times the isthmus is absent,
as normally in certain lower
Mammalia; and sometimes
there is more than one pyra-
midal or ascending process.

§ 352. Thymus.-This body is distinguished from the thyroid by its wide central cavity, and by its diminution of volume or disappearance after early age. In the Human subject, e.g., at birth the thymus, fig. 438, a, a, may weigh 240 grains, and increase to 270 grains in the infant of one year: but, with the development and exercise of the muscular system, it wastes away, and may be reduced at twenty-one years of age to a remnant weighing only forty grains. After twenty-five it is rare, or difficult, to discover any of its structure left in the areolar

1 XII. tom. viii, p. 675.

2 cir'. p. 44. Peters confirms this, in ccx". p. 95, Taf. 4, fig. 5, gl.

At birth the bulk of the gland lies

439

tissue of the mediastinum. behind the manubrium, descending to near the middle of the sternum, and ascending upon the fore and lateral parts of the trachea to the thyroid. By dissection the thymus can be separated into two lateral portions, which are naturally distinct at an earlier phase of development; each lateral part being a narrow elongated body, folded upon itself, and further resolvable into lobules and acini, like those of a true conglomerate gland: but all the acinal cavities communicate with a central reservoir, fig. 439, occupied by a milk-like solution of albuminoid or proteine principles. Formifaction here produces corpuscles, very closely resembling (in fact identical with) the nuclei of glandular cells; but presenting more numerous nucleoli: their form being for the most part spherical. Mingled with these I have found in the thymus of a Calf, as well as in that of a young Guinea-pig, a few larger corpuscles, about double the size of the former, of spherical form, filled either with a granular matter alone, or containing also a nucleus, or larger vesicular body.'2

3

The thymus in Monotremes lies between the episternum and the beginnings of the vessels from the aortic arch. In a Kangaroo from the pouch Simon found the thymus on the pericardium with a medial lobe besides the two lateral ones. In Rodents the thymus consists of two long lobes extending from the base of the heart, parallel with each other, forward, to the root of the neck. Bodies extending from this position to the posterior mediastinum and forward along the cervical vessels to near the mandible, but consisting, according to Simon, of aggregates of fat-vesicles, undergo periodical increase, in the Marmots, prior to hibernation. In a Bat dissected in March, Dr. H. Jones could not detect any certain homologue of a thymus; but found on each side of the root of the neck a yellowish lobulated mass consisting of conical lobes defined by limitary membrane: the lobes were hollow and filled by aggregations of celloid particles, which were not manifestly nucleated, nor provided with an envelope, but consisted of aggregations of oil-drops and molecules. In the subjoined view, fig. 440, of a portion of this body, magni1 CCXIV". p. 1093. 4 CCXIV". p. 1096.

4

2 Ib.

3 CCXV".