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  Section: Anatomy of Vertebrate Animals » The Classification and Organization of the Mammalia
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The Platyrrhini


2. The Platyrrhini are essentially quadrupedal and plantigrade, though some, like the Spider Monkeys (Ateles), occasionally assume the erect posture. They all possess tails, and in some genera (e. g., Ateles) this organ becomes very flexible and muscular, and the under surface of its extremity is devoid of hair and highly sensitive. The tail, thus modified, is a powerful prehensile organ, and serves as a fifth hand. The partition between the nostrils is broad and separates them widely, so that the nose is remarkably wide and flat, whence the name of the group. The ears are rounded and bare. There are no cheek-pouches, nor ischial callosities, in any Platyrrhine Monkey. In most, the fore-limbs are shorter than the hind-limbs, but the reverse is the case in the Spider Monkeys The pollex differs less from the other digits than it does in the Catarrhini. It is more nearly parallel with, and in the same plane as, the other digits of the manus; and, though capable of extensive adduction and abduction, can hardly be said to be truly opposable. The hallux is large, and susceptible of extensive movements in abduction and adduction.

The number of the dorso-lumbar vertebrae varies from seventeen to twenty-two, the greatest number being possessed hy Nyctipithecus, which has 22 (14 + 8 or 15+7). In those forms which have prehensile tails the terminal caudal vertebrae are flattened from above downward. The articular surface of the head of the humerus looks more backward than inward; and, not unfrequently, there is a foramen above the inner condyle. The carpus contains nine bones. The pollex is generally complete, but, in Ateles, it is reduced to a small metacarpal (to which, usually, a single minute nodular phalanx is articulated), and is completely hidden beneath the integument. The pelvis is, generally, elongated, and the anterior ramus of the pubis lies at right angles with the long axis of the narrow ilium. The tuberosities of the ischia are everted, but not rugose. In Ateles, the pelvis is broader, and the pubis forms a more open angle with the ilium. The calcaneal process is always very short, and compressed from side to side.

The brain-case is rounded and devoid of strong crests. There is no distinct mastoid process, and the styloid is not ossified. The coronal suture is generally V-shaped, the apex of the frontal bone extending far back on the vertex of the skull. The alisphenoid and the parietal bones unite upon the side-walls of the skull. The external auditoiy meatus is not ossified, the tympanic bone retaining its foetal, hoop-like form. The frontal bones approach one another on the floor of the skull, but rarely unite over the junction of the presphenoid with the ethmoid.
On the inner surface of the periotic bone there is a fossa overarched by the anterior vertical semicircular canal, in which the flocculus rests. In Ateles the greater part of the tentorium is ossified. In other respects, the skull presents extraordinary variations among the Platyrrhini; the two extremes being presented by the Howling Monkeys (Mycetes) and the Squirrel Monkeys (Chrysothrix). In the former, the face is very large and prominent, with a low facial angle. The roof of the brain-case is depressed; the plane of the occipital foramen is almost perpendicular to the basi-cranial axis; and that of the tentorium is very much inclined. The occipital condyles are, consequently, situated at the posterior end of the basis cranii, and the basi-cranial axis is as long as the cerebral cavity. In Chrysothrix, on the contrary, the face is relatively small, with a high facial angle; the braincase is moderately arched; the plane of the tentorium is horizontal, like that of the occipital foramen, which lies but little behind the middle of the base of the skull. The basi-cranial axis is much shorter than the cerebral cavity. The premaxillo-maxillary suture disappears early in Cebus.

The formula of the adult dentition is i. 2.2/2.2 c. 1-1/1-1 p.m. 3.3/3.3 m. 3.3/3.3=36. The crowns of the molar teeth usually have two transverse ridges, ending in four cusps. In the upper molars of Ateles and Mycetes an oblique ridge crosses the crown from the antero-external to the postero-internal cusp. The permanent canines usually make their appearance before the last molar.

The stomach is simple, the caecum large, and devoid of any vermiform appendix; the liver is usually five-lobed; and the kidney has a single papilla.

The ventricles of the larynx are not usually developed into air-sacs. In Ateles, however, a median air-sac is developed from the posterior wall of the windpipe between the cricoid cartilage and the first ring of the trachea. A very remarkable modification of the hyoid and larynx takes place in Mycetes. The cornua of the hyoid are rudimentary, but its body is converted into a large thin-walled bony drum, the cavity of which communicates, beneath the large epiglottis, with that of the larynx. The thyroid cartilage is very large, and the cartilages of Wrisberg and Santorini are replaced by a fibrous mass, which is united posteriorly with its fellow of the opposite side. In addition to the hyoidean air-sac the ventricles of the larynx are dilated and prolonged upward, coming into contact above the larynx; two pharyngo-laryngeal pouches may be added to these. Mycetes is famous for the distance to which its howling voice can be heard in the South American forests.

Although the pollex is rudimentary and apparently functionless in Ateles, all its characteristic muscles (abductor, adductor, flexor brevis, and opponens) are present, except the long flexor.

In Nyctipithecus the pedal interossei are flexores breves, and lie on the plantar surfaces of the metatarsal bones, as in the Marmosets; but both the adductor hallucis and the transversus pedis are well developed.

The brain varies remarkably in different Platyrrhini. In Chrysothrix, the cerebral hemispheres project beyond the cerebellum to a greater relative extent than in any other Mammal, namely, by one-fifth of their total length. On the other hand, in Mycetes, the cerebral hemispheres hardly hide the cerebellum, when the brain is viewed from above.

In Cebus, the outer surface of the brain is almost as much convoluted as in the Catarrhine Apes. Ateles has the external perpendicular fissure almost obliterated by the annectent gyri, and, so far, exhibits a higher type of brain than the Catarrhini; but, in Pithecia, Chrysothrix, and Nyctipithecus, the external sulci gradually disappear, until the brain is almost as smooth as in the Marmosets. On the inner faces of the hemispheres, however, the internal perpendicular, the calloso-marginal, the calcarine, and the collateral sulci remain, while, in the interior of the hemispheres, the posterior cornu and the hippocampus minor are always present.

The vermis of the cerebellum is large and projects beyond the level of the posterior margins of its hemispheres; the flocculus is large and lodged in a fossa of the periotic ossification, as in the Marmosets. The upper ends of the pyramids are separated by corpora trapezoidea from the pons Varolii.

The penis is usually terminated by a large, button-shaped glans. The cavity of the tunica vaginalis is not shut off from the abdomen, and the testes lie at the sides of, rather than behind, the penis. The female Ateles has a long clitoris, which depends from the vagina.

The Platyrrhini occur only in the Austro-Columbian province, and are known in the fossil state only in certain caves of that region.


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