1902 Encyclopedia > Ape > Ape Anatomy - Introduction. The External Form and Clothing.

Ape
(Part 15)


(C) The Anatomy of Apes

Ape Anatomy - Introduction

To describe generally the anatomy of apes would be, in fact, to describe in an elementary manner the general anatomy of man. It is necessary, therefore, here to assume that the reader has already an elementary knowledge of human anatomy, and to refer such readers as have not such knowledge to the article ANATOMY.

Ape Anatomy - The External Form and Clothing

The difference in bulk between the different members of the group (e.g. Troglodytes and Hapale) is extreme, being greater than that between a man and a common squirrel.

The proportions of the body as regards the relative lengths of the two pairs of limbs one with the other, and both with that of the trunk, vary considerably. Both pairs may be much elongated, as in Ateles and Hylobates, and either sub-equally, as in the first of these genera, or with the arms very greatly in excess, as in the second. The legs may be excessively short, and the arms, at the same time, excessively long, as in the orang. Both pairs may be short and sub-equal, as often in the Cynopithecinae. Only in the Nyctipithecinae and Hapalinae does the excess in length of the lower limbs over the upper exceed or equal that which is found in man. The length of the tail presents some noteworthy points. At its first appearance it is found at once at its greatest absolute length, and also greatly developed relatively, being about twice the length of the trunk. Its greatest relative length is, however, attained in Ateles, where it reaches three times the length of the trunk. The constancy of the degree of its development varies much in different groups. In the greater number of genera it is long all the species, and in some (Simia, Troglodytes, and Hylobates) it is absent in all. In others it may be long or short, or completely absent, e.g. in Macacus. The form of the head presents great differences -- it may be rounded, as e.g., in Ateles; produced vertically, as in Simia; drawn out posteriorly to an extreme degree, as in Chrysothrix; or anteriorly, as in Cynocephalus. A production of the muzzle, necessitated by the presence of large teeth, exists already in Troglodytes; but in the baboons, not only is this prolongation carried much further, but the terminal position of the nostrils gives an emphatically dog like aspect to the face.

The eyes may be small compared with the size of the head, as in the baboons, they may, on the contrary, attain a relatively enormous size, as in Nyctipithecus. They are always forwardly directed, and never much more separated one from another than in man. They may, however, be much more closely approximated, as notably in Chrysothrix.

The external ears are always well developed, and have very generally their postero-superior angle pointed. They may be large and small in the same genus, as in Troglodytes. Only in the gorilla do we find present, even in a rudimentary condition, that soft depending portion of the human ear with is termed the "lobule."

The nose has scarcely ever more than a very slight prominence, and yet an enormous development is to be met with in Semnopithecus nasalis; while in S. roxellanae we find a sharply prominent, though smaller and extremely upturned, nose. The hoolock gibbon also possesses a prominent but slightly aquiline nose, the terminal position of the nostrils in Cynocephalus has been just mentioned. These apertures may be closely approximately, as in all the Simiadae, or they may be separated one from the other by abroad septum, as in the Cebidae, its breadth, however, varying somewhat in different genera, as, e.g. in Ateles and Eriodes, and in Callithrix and Nyctipithecus.

The lips are generally thin, but may be very extensive, as in Simia.

The hands are generally provided with thumbs, though these organs (as in Colobus and Ateles) may be represented only by small nailless tubercles. The thumb (pollex) is more human in its proportions in the chimpanzee than in any other of the highest apes. As compared with the length of the hand, it is most man-like in the lowest Cebidae, e.g., Chrysothrix and Hapale. In spite of greater relative, length, however, it may but little merit the name of thumb, as it is but slightly opposable to the other digits in any of the Cebidae, and is not at all so in the Hapalinae.

The hallux (great toe) is never rudimentary like the pollex. It is never, as it often is in man, the longest digit of the foot, but is constantly the shortest one. As compared with the entire length of the foot, it is most human in the chimpanzee and some gibbons, and smallest of all in the orang, and next smallest in Hapale. More detailed proportions will be more appropriately given in speaking of the skeleton.

Every digit is provided with a nail, except the hallux of the orang and those rudimentary tubercles already spoken of as representing thumbs in Ateles and Colobus. The nail of the hallux is flat in every species, but the other nails are never so flat as are the nails of man. The lateral compression of the nails becomes more strongly marked in some Cebidae, e.g., in Eriodes, but attains its extreme in the Hapalinae, where every nail, except that of the hallux, assumes the form of a long, curved, and sharply-pointed claw.

All the apes, without exception, differ from man in having the body almost entirely clothed with copious hair, and especially in never having the back naked. In the gibbons, the Semnopithecinae, and the Cynopithecinae, naked spaces (ischiatic callosities) are present on that part of the body which is the main support in the sitting posture. These naked spaces increase in size as we descend through the series of Cynopitheinae, and are subject to a tumefaction (sometimes excessive and extending to parts adjacent) at the season of sexual excitement. Such naked spaces are never found in any of the Cebidae. No ape has so exclusive and preponderating a development of hair on the head and face as exists in most men.

As to the head, long hair is found thereon in Hapale oedipus and in some of the Semnopitheci, whilst certain of the Macaci (as, e.g., the Chinese bonnet moneky, M. sinicus) have the hair of the head long, and radiating in all directions from a central point on the sinciput. A beard is developed in the male orang, and Cercopithecus Diana has long hair on the cheeks and chin. The wanderoo (Macacus silenus) has the face encircled by a kind of mane of very long hairs, and many of the marmosets have a long tuft of hairs on each side of the head. The American apes exhibit some extremes respecting hair development. Thus in some of the howlers (as in some of the Colobi of the Old World) the hair of the flanks is greatly elongated. Some also have an elongated beard, but the latter structure attains its maximum of development in the couxio (Pithecia satanas). Some of Pitheciinae have the hair of the whole body and tail very long, others have the head of the female furnished with elongated hair, while another species (Brachyurus calvus) has the head bald. Long hair may be developed from the shoulders, as in Cynocephalus hamadryas and Hapale humeralifer; or many form a tuft at the end of the tail, as in Macacus silenus, Cynocephalus hamadryas, and Cynocephalus gelada.

The direction of the hair may sometimes vary in nearly allied forms. Thus the hairs on the arm and forearm respectively may be so directed that the apices converge towards the elbow. Such is the case in most of the latisternal apes, yet in Hylobates agilis all the hair of both these limbs segments is directed towards the wrist.

The hair presents generally no remarkable character as to its structure. It may, however, assume a very silky nature, as in Hapale rosalia, or assume the character of wool, as in Eriodes, and as in that remarkable form recently discovered by Father David, Macacus thibetanus. The last named species inhabits the snowy ranges of the Thibet [Tibet] mountains, and is provided for this habitat by a modification in its hairy clothing similar to that which suited the extinct mammoth for the severity of its Siberian home. This fact as to M. thibetanus has an interesting bearing on fossil forms, which we shall have to consider later.

Great brilliance of colour is sometimes found in the naked parts of the body, particularly in the Simiadae, and especially in the regions of the face and sexual organs. In some of Cercopitheci and Cynocephali, rose colour, turquoise blue, green, golden yellow, and vermilion appear, in various combinations, in one or other or both of these regions, and become especially brilliant at the epochs of sexual excitement.





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