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

base of the heart and beginnings of the great

y 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

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

72

ner ones lying in the axilla.'' 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.

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

6

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.

441

d

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,' bnucleus enwrapped in a fine granular mass,' c cell,' dnuclear 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

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

the Mole the adrenals have the form of a three-sided pyramid. In the Coypu they are long and rounded, of a greyish-yellow externally; their medullary structure like soft liver. Their length was one inch: their situation mesiad of the upper extremity of each kidney. They have a similar cylindrical figure, and large relative size in the Porcupine and many other Rodents: they are shorter in the Muride; are roundish and somewhat flattened in Leporida. In a young Sloth I found the adrenal surpassing the kidney in size and showing distinctly the cortical and medullary substances. In the Cetacea there is an interesting analogy between the adrenal, in regard to its lobulated exterior, and the multilobate kidney. In the Elephant the adrenal is a depressed cone, with the base bilobed. In the Rhinoceros the adrenal bodies, like the kidneys, differed from each other in form; they were elongated and nearly cylindrical. The right had one extremity bent at a right angle: its length in a female Rhinoceros was three and a half inches; its breadth across the bent extremity two inches: the left was simply elongated, three and a half inches long, one and a half broad, and one inch thick. In section they presented an external greyish-yellow fibrous cortex, from one-fourth to onethird of an inch thick, enclosing a fleshy-coloured substance, in the middle of which there was a semilunar portion of the grey fibrous matter: there was no trace of a central cavity. Both suprarenal bodies adhered closely to the contiguous large veins.' In the Horse the adrenals are flattened and triangular. In the Ox they somewhat resemble the kidney itself in shape: in the Reindeer they are a full oval: in the Sheep they are more elongate. In the Seal, as in the Whale, they resemble the kidney in their finely lobulate exterior: in the Dog they are longish and cylindrical: in the Cat roundish and somewhat flattened. In the Aye-aye the adrenals are subtriangular, elongate, depressed, and relatively larger than in the higher Quadrumana, in which the adrenals progressively approach the shape and proportions bryo, eight lines presented in the human subject.

442

α

d

Urogenita organs.

from Human em

long. CCXVII".

In the foetus the adrenal, like the kidney, shows a lobulated exterior: at an early period of the development of these bodies the adrenal, fig. 442, a, exceeds the kidney, b, in size both are preceded by the deciduous or Wolffian kidneys, d, d. In the embryo of the twelfth week the kidneys and adrenals

cn". p. 45, pl. xi, fig. 1, n, n.

are of equal size: in the sixth month the kidneys have gained in weight so as to be as five to two: and at birth they are as three to one: after this time the adrenals diminish so as in the adult to be onlyth the size of the kidney. Occasionally they entirely

waste away.

The large proportional supply of nerves to the mammalian adrenals from the contiguous plexuses (coeliac and renal) of the sympathetic system is worthy of note.

CHAPTER XXXIII.

RESPIRATORY SYSTEM OF MAMMALIA.

354. Lungs of Mammalia.-The class-characteristic afforded by these organs is defined in vol. ii. p. 266, and exemplified in fig. 139, ib. In all Mammals each lung, ib. lg, is conical, with the base resting upon the diaphragm, ib. d, and the apex reaching to the root of the neck: the shape, and especially the degree of subdivision, of the pulmonary cone offer many varieties in the class. The most common quadrupedal difference from the bimanal type is the lobe, called 'azygos' or 'impar,' detached from the right lung to occupy the space between the heart and diaphragm, as at n, fig. 308 (Ornithorhynchus). The outer surface of the lung is smooth, being covered by a serous membrane, reflected from the great blood- and air-vessels forming its 'root' upon the walls of the thorax; thus constituting a shut sac, called pleural,' distinct from that of the opposite lung. The portions of the pleuræ passing respectively from the pulmonary roots to the back and fore parts of the thoracic cavity, are called 'mediastinal,' and intercept the pericardium, great vessels, thymus, gullet, and other parts intervening between the two lungs: the regions of such thoracic septum being defined, in Anthropotomy, as anterior,' 'posterior,' and middle mediastina.' The pleural serous sacs are peculiar to Mammalia: they facilitate the movements of the lung upon the thoracic walls during respiration.

The wind-pipe bifurcates to supply each lung, fig. 418, P, P., with air, as does the pulmonary artery conveying the blood to be affected thereby; the pulmonary veins, ib. p, return the blood so changed to the heart. Besides these three main constituents of the root' of the lung, it includes the bronchial' or nutritive arteries and veins, absorbents and nerves, with their connective tissue, and the enveloping pleural sheath. Beneath the serous covering of the lung is a layer of combined areolar and elastic tissues, the latter predominating in the denser sub-serous' coat of the lungs of the larger carnivorous and ungulate mammals: in Cetacea the smooth contractile fibre is therewith intermixed.

The trachea is kept patent by cartilaginous hoops, the ends