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 chord tendinez 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 (fig. 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- 
ominata, left carotid, and left subclavian. 
; innominata divides, after a course of three 
" . into the right subclavian and carotid 
(fig. 187, i), the latter being the smallest 
nch. Both subclavians emerge from the 
tl above the first rib, and pass between 
it and the coracoid. 
The phrenic, celiac, and mesenteric arteries 
e given off from the abdominal aorta; the 
renal artery is short, wide, and single; there is 
0 inferior mesenteric artery, but the abdominal 
orta terminates by dividing into the two com- 
mon iliac and the caudal arteries, the arterial 
em agreeing in this and the other essential 
acters with the Mammalian type. The 
crural artery is shown at y, fig. 180. 
__ Each of the superior vene cave receives the 
_azygos vein of its respective side. The inferior 
has a long course in the thorax; it is 
, y 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 
renal artery, and communicate solely with 
inferior cava. The vena porte is consti- 
ed as in other Mammalia. 
oP inal 
RESPIRATORY SYSTEM. 
The Jungs of the Monotremata are con- 
to the thoracic cavity, and suspended 
ely in compartments partitioned off by du- 
tures of the pleura. The right lung is 
ided, in the Ornithorhynchus, into three 
lobes, of which the smallest (fig. 187, ») fills 
interspace between the heart and diaphragm : 
@ left lung (0) is undivided. The structure 
the whole is spongy, and divided into 
ute cells. 
e trachea (fig. 187, m) is wide, as in 
aquatic mammals: the cartilaginous 
‘Tings, fifteen in number, are broad and slightly 
_ overlap each other: the bronchial annuli are 
__ bony, and are continued of that texture through 
Bik, part of the lungs. 
_. Inthe Echidna the trachea is narrower than 
in the Ornithorhynchus: there are twenty-two 
__ tracheal hoops, which are disunited behind ; 
: firm cartilaginous annuli are continued 
_ along the larger branches of the bronchus for 
; eel way into the lung, but the smaller 
___ branches are membranous. 
There is no trace of inferior larynx in either 
_* Meckel, 1. c. p. 32. 
MONOTREMATA. 
Monotreme. The superior /a- 
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 prominentand acuminate: 
the lateral ale 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, a) 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 lobulated 
structure, and pretty firm texture. 
RENAL SYSTEM. 
The suprarenal bodies (fig. 190, 6, 6) 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, are 
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 sinall 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 Chelonia ; 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 vesicule seminales. 
