RESPIRATORY SYSTEM.
The lungs of the Monotremata are con fined to the thoracic cavity, and suspended freely in compartments partitioned off by du plicatures of the pleura. The right lung is divided, in the Ornithorhynchus, into three lobes, of which the smallest (fig. 187, n) fills the interspace between the heart and diaphragm : the left lung (o) is undivided. The structure of the whole is spongy, and divided into minute cells.
The trachea (fig. 187, m) is wide, as in most aquatic mammals : the cartilaginous rings, fifteen in number, are broad and slightly overlap each other : the bronchial annuli are bony, and are continued of that texture through a great part of the lungs.
In the Echidna the trachea is narrower than in the Ornithorhynchus : there are twenty-two tracheal hoops, which are disunited behind ; very firm cartilaginous annuli are continued along the larger branches of the bronchus for some way into the lung, but the smaller branches are membranous.
There is no trace of inferior larynx in either Monotreme. The superior la rynx is conformable to the Mam malian type, but presents some remarkable modifications in the Ornithorhynchus. The thyroid
cartilage (fig. 189, c) in this animal is very broad; its middle part is prominent and acuminate : the lateral aim are bony,and each of them divides, and sends one of the processes to the posterior part of the pharynx (fig.186, c), where it becomes cartilaginous, and is confluent with the corres ponding process of the opposite side. The cri coid cartilage (fig. 189, d) is ossified at its middle anterior part. The arytenoid cartilages, (fig. 189, e, e) present the usual triangular form, and are of large size. The epiglottis (fig. 189, a) is remarkably broad, with an acuminated and notched apex.
Besides a small thymus gland, Meckel found in the Ornithorhynchus two otherlateral glands on the external part of the chest, extending be tween the scapula and humerus, covered only by the panniculus carnosus and the trapezius. These presented a reddish colour, a lobulated structure, and pretty firm texture.