COMPARATIVE ANATOMY.-I pass on now hastily to notice such deviations from sym metry as are met with in the normal conditions of the lower animals belonging to the ver tebrate sub-kingdom, and to examine the question of symmetry in the other sub-king doms.
In all Mammalia there is much the same departure from symmetry in the viscera of the chest and abdomen as is found in the human subject ; but in no other mammal is the lateral displacement so great as in man, for in all others there is a greater proportionate antero posterior depth of chest and belly. The only other notable instances of a-symmetry known to exist in the mammalian class are the fol lowing: — In the male narwhal or sea-unicorn the left front incisor tooth attains the enormous length of eight or ten feet, while the right one is found as a rudiment that never pierces the gum. Spirals in the middle line are departures from symmetry ; consequently the spiral penis of the boar, &c. must be regarded as instances of asymmetry; a slight excess in length of one of the halves is sufficient to produce this spiral form. The left nostril of most of the Ce tacea is constantly much larger than the right.
In Birds there is much the same want of symmetry in the viscera of the trunk as is met with in mammals. The right one of the se cond pair of embryonic aortic arches however is retained instead of the left, so that the adult aorta arches over the right bronchus. But notwithstanding this, the left ventricle is the systemic one, and presents the same excess over the right, in the thickness of its walls, as in Mammalia. Both of the ductus Cuvieri are retained and form two superior yew came. The liver is situated mesially, that is to say, its great fissure and falciform ligament are in the middle line ; but its lift lobe is usually the larger, and in the common fowl presents a fissure which is not repeated in the right. The oesophagus diverges slightly towards the right, but the cardiac orifice of the stomach is to the left of the pyloric. The long loop described by the duodenum, and the pancreas which is surrounded by it, are found extending diagon ally across the abdomen in front of the other bowels, their extremity resting in the left iliac i fossa, but they are not fixed in this position.
The gall bladder is situated as in Mammalia. There being no great omentum, the spleen occupies its typical position behind the sto mach. There is usually a disparity in the length of the pair of caeca met with in this class.
But the most remarkable exception to sym metry in the class Ayes is that which exists in the femalegenerative organs. The left ovary and oviduct alone are functionally evolved, whilst the right, becoming atrophied at an early period, are barely traceable in the adult ani mal. A few instances are on record where these right lateral homologues of the ovary and oviduct have been fourrd evolved in func tional size as a testicle and vas deferens, thus forming lateral hermaphrodism.
The male generative organs of birds are all symmetrical except the penis, when it exists, which is spiral.
Another instance of want of symmetry is presented by the beak of some birds, as the cross-bill, &c.
Reptiles. — The heart of reptiles is situ ated in the middle line, but it is not symme trical in form, nor do the great blood-vessels enter and quit it in a perfectly symmetrical manner ; they however approach more nearly to symmetry in this class than in any of those which have been previously considered. In all reptiles there are at first two aortm, sym metrically disposed, arching over the right and left bronchi respectively, and uniting with one another to form one trunk on the spine. The vessels given off from these are, however, most generally not symmetrical, the head and front limbs being supplied frequently from the right arch alone, and the chylopoietic viscera from the left alone. The pulmonary arteries arise behind the origin of the aortm. The lungs of reptiles are usually two symmetrical organs, but in the Ophidia the left lung, when it exists, is much shorter than the right, and in some, as Coluber thiringicus, it is wanting altogether, the only vestige of it being a cmcal depression on the left side of the lower end of the trachea ; this absence of the left lung entails, of course, the loss of the left pulmonary vessels.