368 THE MECHANICAL FUNCTIONS. 



branches, or sportively swinging by their extremities in the 

 air. Both the feet and the hands are formed for this species 

 of prehension; and many are farther provided with a strong- 

 ly prehensile tail, which is an instrument admirably adapted 

 for all these purposes. Hence, the attitude most natural to 

 these animals is neither the horizontal one of quadrupeds, 

 nor the erect posture of man, but an intermediate or semi- 

 erect position. 



This view of the living habits of the quadrumana will af- 

 ford the key to most of the peculiarities of structure they 

 present to our observation. The head, being no longer sus- 

 pended at the end of a horizontal, or recurved neck, is, in 

 the usual attitude of the animalj supported chiefly by the 

 cervical vertebrae. The greater development of the brain, 

 and, miore especially, of its posterior lobes, creates a neces- 

 sity for an extension of the occipital bone in that direction, 

 a portion of the weight to be sustained by the atlas, is, ac- 

 cordingly, thrown behind the centre of motion, which is at 

 its articulation with the latter bone; and this weight tends, 

 therefore, to balance that of the anterior part of the head; 

 hence, there is no need of the strong cervical ligament, which 

 is so universally met with in quadrupeds, and, although this 

 ligament exists in the monkey, it is very slender, and of no 

 very great extent. 



Great mobility has been conferred on the spine by the 

 form of its articulations; and the caudal vertebrae are gene- 

 rally greatly multiplied to form a tail of considerable length, 

 which, in the Jiteles, or spider monkey of America, is 

 moved by powerful muscles, and is an organ of great flexi- 

 bility and strength. Monkeys possess a distinct clavicle, a 

 lengthened humerus and a femur, a radius and ulna movea- 

 ble upon each other, and a hand nearly approaching to the 

 human construction. But the thumb is less developed, and 

 its muscles are much weaker than in man. 



The bones of the pelvis, as well as those of the leg, are 

 elongated, for the purpose of giving greater length to the mus- 

 cles which are to move their several parts; by this means, 



