It is freqnently easier to perceive, as it were intuitively, the distinctive charac ters of two neighbouring species of ani mals, than to express them by words. Hence Linnzus, whose sagacity in per ceiving the characteristic marks of the various objects of natural history, and in expressing them in appropriate language, has never been exceeded, declares, in his " Systema Natures," that the distinctions between man and the monkey still remain to be discovered : " Mirum, adeo parum differre stuttissimam simiam a sapientissi mo homine, ut iste geodmtes naturx etiam num qu2erendus, qui hos limitet." Accord ingly, he gives neither the generic nor spe cific character of man in that work.
The cirumstances which distinguish man from other animals may be consider ed under three divisions : 1. Differences in the structure of the body ; 2. in the animal economy ; 3. in the faculties of the mind.
Under the first head we remark, as the most distinguishing peculiarity of man, his erect stature: that majestic attitude, which announces his superiority over all the other inhabitants of the globe. He is the only being adapted by his natural formation to the upright position. Enslaved to their senses, and partaking merely of physical enjoyments, other animals have the head directed towards the earth : " (lux natu ra prona atque ventri obedientia finxit." Man, whose more elevated nature is con nected to surrounding objects by moral relations, who can embrace in his mind the system of the universe, and follow the connections of effects and causes, boldly regards the heavens, and can direct his sight even into the starry regions. The physical cause of this noble prerogative will be found in the length and breadth of the feet ; in the length and strength of the lower extremities; and in the num ber and size of the muscles, which extend the trunk upon the lower limbs. (For.a more detailed account of this part of the subject, see COMPARATIVE ANATOMY, muscles.) The situation of the great occipital fora. men is another circumstance depending on the erect stature of man : and for an account of this subject, we refer to the same part of the article on comparative anatomy, and also to that portion of it which treats of comparative osteology.
The structure of the thorax chews, that man was nut designed to go on all-fours. Quadrupeds, if they have long legs, have the chest flattened at the sides, and keel shaped in front ; and they have no clavi cles, so that the front legs converge, and fall under the chest, to support the front of the body. Quadrupeds have also a longer sternum, or a greater number of ribs continued towards the crista ilia, and serving the purpose of supporting the ab dominal viscera in the horizontal position of the trunk. These things are all differ
ently arranged in the biped man. His thorax is flattened before and behind ; his shoulders widely separated from each other by the clavicles ; his sternum short, and his abdomen unfurnished with bony parietes in a very large extent. These cir cumstances,with many others, which could not fail to strike any body who attentive ly compared the human skeleton with that of the long-legged quadrupeds, skew how ill the human structure is adapted to progression on four feet, which could not be . otherwise than unsteady, troublesome, and fatiguing, in the highest degree.
The manner in which the human pelvis differs from that of all other animals, is a further proof of what has been already stated.. The broad expansion of the up per part of the ilia forms a firm basis for the trunk ; the curvature of the sacrum, and the inclination of the os coccygis for wards, which is a circumstance altogether peculiar to the human pelvis, give to it a capacity exceeding that of any other ani mal. In the orang-outang the upper part of the ilium is narrow and elongated, stretching upwards in the direction of the spine ; the sacrum, flat and contracted, continues in a straight line with the ver tebral column. • The relation of the neighbouring soft parts to the pelvis, deserves also to be considered. The posterior surface of the pelvis gives origin to the glutxi muscles, the external of which, exceeding in size all others in the body, and covered by a large proportion of flu, form the buttocks. These fleshy and rounded prominences, between which the anus is deeply hidden, have always been considered, both by the natural historian and the physiologist, as a peculiar characteristic of man, particul arly distinguishing him from the simix, which have no buttocks at all.
The curvature of the sacrum and os coccygis gives rise to the particular di rection of the organs of generation, and. especially of the vagina ; that canal, which, in the other female mammalia, nearly follows the axis of the pelvis, be ing placed almost at right angles to that axis in the woman ; and hence the pro cess of parturition becomes more difficult. In consequence of this direction of the vagina, the human female is not like that of brutes, retromingent : and the same circumstance will determine a point that has been often agitated, concerning the most natural position for the act of copu lation : " quibus ipsa modis tractetur blanda voluptas." For although there are many ways in which this rite may be performed, the relation of the penis to the vagina points out the ordinary method as the most natural.