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 Delphinida. 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

1 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.

a

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, lie 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

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 'pyramid or mesial column,' which in some subjects reaches to the hyoid bone. Many varieties have been noted in the human thyroid. Sometimes the isthmus is absent, as normally in certain lower Mammalia; and sometimes there is more than one pyramidal 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;

of

age to

but, with the development and exercise of the muscular system, it wastes away, and may be reduced at twenty-one years 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 ci'. p. 44. Peters confirms this, in ccxi". p. 95, Taf. 4, fig. 5, gl.

439

tissue of the mediastinum. At birth the bulk of the gland lies 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

6

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.3 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, magni'CCXIV". p. 1093. 4 CCXIV". p. 1096.

2 Ib.

3 CCXV".

fied, only the peripheral row was visible, the rest of the mass being opaque. In terrestrial Insectivora the thymus is less ambiguous, and consists of two nearly equal lobes lying on the

440

Convexity of lobe of cervical thymus-like body, Bat. XCXIV".

base of the heart and beginnings of the great vessels. In the Hedgehog were found two roundish masses almost precisely similar to those in the same situation in the Bat, and two broader and thincelloid particles were

ner ones lying in the axilla.'1 The more loaded with oil than in the Bat, and in some parts they were more or less broken up and the oily matter diffused in the cavity. In both cases these lobulated masses may be wellmarked modifications of the adipose tissue. In Cetacea a thymus has been recognised in Balana mysticetus, the right lobe extending over the aortic arch to the trachea, where it terminates in two small cornua, the left lobe being of smaller size. In the foetal Dolphin these are two large median portions, pericardiac and tracheal, with deep-seated lateral cornua.'3 In a fœtal Elephant the thymus is a flat mass beneath the anterior part of the pericardium, with a short forward prolongation of the right lobe. In the Rhinoceros the thymus holds a like position, and encroaches but a little way upon the neck. In the Artiodactyles, whether ruminant (Ox, Deer) or non-ruminant (Peccari), the cervical portions of the thymus are more developed, often extending to the mandibular angles. The thymus of the Calf is very large and affords a good subject for investigating the structure of this body. In Carnivora the thymus has the usual position in the thorax, to which it is limited; it soon shrinks, and in Felines disappears. At its fullest phase of development in the Cat, the thymus is thick from before backward, and its right and left lobes closely interdigitate. In a young Seal, Simon found it in two symmetrical, broad, thickish lobes, extending to the root of the neck, and abruptly terminated by clubbed extremities, which are deeply grooved in front by the left vena innominata." In most Quadrumana, especially the Catarhine group, the thymus presents the same general shape and relations as in the human subject.

6

§ 253. Adrenals.-These bodies are best developed in Mam

1 CCXIV".

p. 1096.

2 Ib.

3 CCXV".

4 Ib.

malia; and, in the Bimanous order, they repeat, though in a minor degree, the relation of largest relative size to the immature period. They are subtriangular, flattened, with their base excavated and resting, in Man, upon the upper end of the kidney, whence they have been termed supra-renal capsules:' in lower Mammals they are more commonly mesiad of the upper end of the kidney, and not always in contact therewith: at the base of the part is a fissure giving issue to the large adrenal vein. The substance of the body is distinguished by, usually conspicuous, differences of colour into cortical' and 'medullary;' the former, in Man, being yellowish-brown, the latter reddish-brown: the cortical substance is also firmer than the medullary, which receives more blood, and appears soon to dissolve after death, occasioning the cavity there usually found. The proper areolar capsule sends incurved processes, localising the tissue into lobes and lobules: the ultimate texture of the cortical substance being minutely vesicular, the vesicles varying in size, but affecting an arrange ment in rows. The vesicles are smallest at the limits of the medullary substance, and here inclose spaces in which the usual results of formifaction more especially are met with; such as fine granules, globules, nuclei, and nuclear structures, affording ample ground for misinterpretation as transitions to cell-development' and metamorphosis to the cell-form,' &c.

Ecker has delineated some of the evidences of size-limiting, form-giving forces, analogous to those of crystallisation, in fig. 441, where a is a nucleus,' b nucleus enwrapped in a fine granular mass,' c' cell,'

6

e

441

d

Forms assumed by proteine matters, in solution within the cellular spaces of the adrenal; Man CCLXXXVII.

d nuclear vesicle of an embryo,' e two gland-vesicles with their contents.' With these are mixed oil-globules; in greater abundance in the adrenals of Lissencephala and Carnivora than in those of Man, and more or less obscuring the nuclei' and 'gland-vesicles.' These, in the Horse, are smaller and more spherical at the periphery, larger and more oval toward the centre, of the cortical substance, there offering the linear arrangement. Gland-vesicles also occur in the medullary substance of the Horse's adrenals. In the Ox the trabecular tissue of the cortical substance defining the lobules is firm and well-marked: the fatty globules are fewer than in Man. The gland-vesicles are distinct in the adrenals of the Hedgehog. In