With the exception of the man-like apes and gibbons and the
Barbary ape (Macacus inuus), the caudal vertebrae of monkeys
exceed four in number; but the mandril, Papio (Maimon) maimon,
has sometimes only five. The short-tailed macaques and uakaris
have from about fifteen to seventeen, the shortness of the tail
being occasioned rather by a diminution in the size of the component
vertebrae than by a decrease in number. In the other forms
the number varies between twenty and thirty-three, the latter being
the number attained in the spider-monkeys (fig. 5). The proportion
borne by this region of the spine to the more anterior parts is greatest
in the spider monkeys of the genus Ateles, almost three to one; in the
other longest-tailed genera it is rarely so large as two to one. The
absolute length of the tail is greatest in the langurs and guerezas,
where also the individual caudal vertebrae attain their greatest
length, namely two inches. The caudal vertebrae generally increase
in length from the sacrum till about the seventh, eighth or ninth,
which, with the tenth and eleventh, are the longest in most
long-tailed forms. In Ateles the eleventh, twelfth, thirteenth and
fourteenth vertebrae are the longest. In most members of the
sub-order the breast bone or sternum is narrow, and consists of a
more or less enlarged upper portion, or manubrium, followed by a
chain of sub-equal elongated bones from three to six in number.
In man, man-like apes and gibbons there is, however, a broad
sternum; or one consisting of a manubrium, followed by one bone
only, as in Hylobates. In the orang-utan the breast-bone long
remains made up of ossifications arranged in pairs, side by side,
successively. The true ribs are seven in number on each side in
the highest forms, but in Hylobates there are sometimes eight; in
Ateles there are sometimes nine pairs; in Hapale the number varies
from six to eight, and from seven to eight in the other genera.
The “angles” of the ribs are never so marked as in man; most so
in Hylobates. Pithecia is distinguished by the greater relative
breadth of the ribs. In no ape or monkey is the thorax half as
broad again as it is deep from back to breast. Nevertheless, in
the Simiidae and Hylobatidae, its transverse diameter exceeds
its depth by from about one-fourth to a little under one-third of
the latter. In Ateles (and sometimes also in Alouata) the thorax is
wider than deep, but in the rest it is deeper than wide.
The greatest absolute length of the fore-limb occurs in the gorilla
(fig. 6) and the orang-utan. The humerus never has a perforation
Fig. 6.—Skeleton of the
Gorilla (Anthropopithecus
gorilla), to exhibit the flattened
sternum, the broad and shallow
thorax, and the great length of
the fore-limbs.
(entepicondylar) on the inner side
of its lower extremity. Except in
the man-like apes, the ulna articulates
with the wrist (carpus). The
hand is capable of pronation and
supination on the fore-arm; and
except in man, the chimpanzee and
the gorilla there is a centrale in the
carpus. The phalanges are the same
in number in apes and monkeys
as in man, except that in Ateles
and Colobus the thumb may have
but one small nodular phalange or
none. The phalanges are generally
more curved than in man, and,
except in the Hapalidae, the
terminal ones are flattened from
back to front. In the Hapalidae
they are laterally compressed,
curved, and pointed to support the
claws characteristic of that family.
The length of the thumb with its
metacarpal bears a much greater
proportion to that of the spine in
Hylobates and Simia than in man.
With the exception of Ateles and
Colobus, the shortest thumb, thus
estimated, is found in Nyctipithecus
and Chrysothrix.
The hind-limb, measured from the summit of the femur to the tip of the longest digit, is absolutely greatest in the gorilla, and then in the orang-utan and the chimpanzee. If the foot be removed, the leg of the chimpanzee is longer than that of the orang-utan. The ankle, or tarsus, consists of the same seven bones as in man, and these bones are so arranged, or bound together by ligaments, as to form a transverse and an antero-posterior arch. In no ape or monkey, however, do the lower ends of the inner metatarsals form the anterior point of support of the antero-posterior arch, as in man. The calcaneum, except in the gorilla, is shorter compared with the spine than in man. The phalanges of the foot are the same in number as in man, except that the great toe of the orang-utan has often but one. They are very like their representatives in the hand, and are convex above, concave and flattened below. Only in the Hapalidae are the terminal phalanges laterally compressed instead of flattened. The toes are never nearly so short relatively in apes and monkeys as in man; yet the proportion borne by the great toe, with its metatarsal, to the spine closely approximates in the gorilla to the proportion existing in man, and this proportion is exceeded in Hylobates and Ateles.
Omitting all reference to the muscles, we find that in apes and monkeys the absolute size of the brain never approaches that of man; the cranial capacity being never less than 55 cub. in. in any normal human subject, while in the orang-utan and chimpanzee it is but 26 and 27½ cub. in. respectively. The relative size of the brain varies inversely with the size of the whole body, as is the case in warm-blooded vertebrates generally. The hemispheres of the brain are almost always so much developed as to cover over the cerebellum, the only exceptions being the howlers and the siamang (Hylobates syndactylus). In the latter the cerebellum is slightly uncovered, but it is considerably so in the former. In Chrysothrix the posterior lobes are more largely developed relatively than in man. As in mammals generally, much convoluted hemispheres are correlated with a considerable absolute bulk of body. Thus in Hapale (and here only) we find the hemispheres quite smooth, the only groove being that which represents the Sylvian fissure. In Simia and Anthropopithecus, on the contrary, they are richly convoluted. A hippocampus minor is present in all apes and monkeys, and in some Cebidae is larger relatively than in man, and absolutely larger than the hippocampus major. Of all apes and monkeys the orang-utan has a brain most like that of man; indeed it may be said to be like man's in all respects save that it is much inferior in size and weight, and that the hemispheres are more symmetrically convoluted and less complicated by minor foldings. The human brain, as known by European specimens, has been supposed to differ from that of apes and monkeys by the absence of the so-called simian fold (Affenspalte) on the posterior portion of the main hemispheres. On studying a large series of Egyptian and Sudani brains, Professor G. Elliot Smith finds, however, that this simian fold, or sulcus, can be distinctly recognized. “It is easy,” he writes, “to select examples from the series of Egyptian and Sudanese brains in my possession, in which the pattern formed by the occipital sulci on the lateral surface of the hemisphere in individual anthropoid apes is so exactly reproduced that the identity of every sulcus is placed beyond reasonable doubt. . . . And if we take individual examples of gorilla-brains, it becomes still easier to match the occipital pattern of each of them to numerous human brains. . . . It is easy to appreciate the difficulties which have beset investigators of European types of brain, and to understand the reasons for the common belief in the absence of the supposed distinctly simian sulci in the lateral aspect of the occipital region of the human brain.”
In no ape or monkey does the series of teeth form so perfect an arch as in man, the opposite series of cheek-teeth tending to become more parallel. None has the teeth placed in one uninterrupted series in each jaw, as is the case in the human species; but there is always a small gap between the upper canine and the adjacent incisor, and between the lower canine and the adjacent premolar. This condition is due to the excessive size of the canines, the interspaces giving passage to the tips of these teeth. This prolongation of the canines into tusk-like weapons of offence and defence (especially developed in the males) makes a great difference between the aspect of the dentition in apes and man. The number of the teeth is the same as in man in all Old World Primates. The New World Cebidae have an additional premolar on each side of each jaw, while the Hapalidae have a molar the less. The incisors are nearly vertical, save in Pithecia and its allies, where their tips project forward. The canines are considerably longer than the incisors, except in Hapale, where the lower incisors equal them in length. The premolars differ structurally from the molars much as in man, except that the first lower one may be modified in shape to give passage to the upper canine, as in the baboons. The grinding surface of the molars consists generally of two incomplete transverse ridges, the end of each ridge projecting more than the intermediate part, indicating the position of the four original tubercles. In the man-like apes there is, however, in the upper molars a ridge running obliquely from the front inner tubercle, or cusp, outwards and backwards to the hind outer tubercle. In the Cercopithecidae this ridge is wanting, but it reappears in Ateles and Alouata amongst the Cebidae. In the Hapalidae the tubercles of the molars are more produced and sharp-pointed, in harmony with the insectivorous habits of the marmosets. The last lower molar may be reduced or much enlarged as compared with the others. Thus in Cercopithecus talapoin it has but three tubercles, while in the macaques and baboons it is very large, and has five well-developed cusps. The number of milk-teeth is as in man, except that American monkeys have an additional one. In general the canines are the last teeth to be cut of the permanent dentition, their cutting sometimes causing such constitutional disturbance as to produce convulsions and death. In the gibbons, however, the canines accompany, if they do not precede, the appearance of the hindmost molar, while in the orang-utan they at least sometimes make their appearance before the latter.
The stomach is simple in all apes and monkeys except langurs,
