MONOTREMATA. 



391 



circumference, and a smaller portion closing the 

 outer angle ; the free margin of the valve is 

 attached to the extremity of a large fleshy 

 column, arising by different roots from both 

 the fixed and the free walls of the ventricle ; a 

 short fleshy column is attached to the left ex- 

 tremity of the valve; some chordae tendine;E are 

 fixed to the right angle of the valve. The rest 

 of the structure of the heart corresponds with 

 that in the Ornithorhynchus. 



The aorta (Jig. 187, d) bends, as in the 

 Mammalia, over the left bronchus. The pri- 

 mary branches come off from the arch, in 

 both Monotremes, as in Man, viz. arteria in- 

 nominata, left carotid, and left subclavian. 

 The innominata divides, after a course of three 

 lines, into the right subclavian and carotid 

 (fig. 187, i), the latter being the smallest 

 branch. Both subclavians emerge from the 

 thorax above the first rib, and pass between 

 it and the coracoid. 



The phrenic, cceliac, and mesenteric arteries 

 are given off from the abdominal aorta; the 

 renal artery is short, wide, and single; there is 

 no inferior mesenteric artery, but the abdominal 

 aorta terminates by dividing into the two com- 

 mon iliac and the caudal arteries, the arterial 

 system agreeing in this and the other essential 

 characters with the Mammalian type. The 

 crural artery is shown at y, fig. 180. 



Each of the superior venae cavae receives the 

 azygos vein of its respective side. The inferior 

 cava has a long course in the thorax ; it is 

 greatly dilated in the liver in the Ornithorhyn- 

 chus, as it is in the Placental divers, the Otter 

 and Seal for instance.* 



The veins of the kidney are continued from 

 the renal artery, and communicate solely with 

 the inferior cava. The vena portae is consti- 

 tuted as in other Mammalia. 



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, ) 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 



* Meckcl, 1. c. p. 32. 



Monotreme. The superior la- Fig. 189. 

 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 alae are bony, and each 

 of them divides, and sends one 

 of the processes to the posterior Larynx of 

 part of the pharynx (fig. 186, c), Ornithorhyn- 

 where it becomes cartilaginous, chus. 

 and is confluent with the corres- (Meckel.) 

 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, ) is remarkably broad, with an 

 acuminated and notched apex. 



Besides a small thymus gland, Meckel found 

 in the Ornithorhynchus two other lateral 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 tabulated 

 structure, and pretty firm texture. 



RENAL SYSTEM. 



The suprarenal bodies (fig. 190, b, b) are of 

 moderate size, of the usual structure, and have 

 the ordinary situation internal to the anterior 

 extremities of the kidneys. 



The kidneys (a, a), in both Monotremes, arc 

 smple, compact, conglobate glands, situated, 

 as usual, far forwards on the loins, the right a lit- 

 tle in advance of the left. The external surface, 

 after the removal of the capsule, is smooth. 

 The renal tissue consists of the two usual por- 

 tions ; the cortical, or softer and more vascular 

 part, being easily distinguishable from the more 

 compact medullary part. The tubuli uriniferi 

 terminate on the concave surface of a small and 

 simple pelvis. 



The ureter (fig- 190, c, c) takes the usual 

 course to the contracted neck of the bladder, 

 but terminates, in the male, in the urogenital 

 canal, below the vasa deferentia; and, in the 

 female, (fig. 191, /,/,) beyond the uterine orifice, 

 which thus intervenes between the ureter and 

 the orifice of the urinary bladder. 



In all respects, save the place of termination 

 of the excretory ducts and their relation to the 

 reservoir of the renal secretion, the Mono- 

 tremes adhere closely, in regard to their urinary 

 system, to the Mammalian type. The circum- 

 stances in which they deviate from the higher 

 mammals approximate them closely to the 

 Reptilia, and especially the Chelonla ; and it 

 is to be observed that the deviation commences 

 where the urinary system begins to be connected 

 with the generative organs, in which the ovipa- 

 rous type of structure is especially manifested. 



ORGANS OF GENERATION. 

 The male organs in both Monotremes con- 

 sist of a testis, vas deferens, Cowper's glands, 

 and penis : there are neither prostatic glands 

 nor vesiculae seminales. 



