S94 THE ANATOMY OF VERTEBRATED ANIMALS. 



there is a complete set of deep extensors. The four dorsal and 

 three palmar interossei are not distinctly subdivided, but they 

 send slips to the extensor tendons. 



There are ionv j^eronoei : p. longus^p. hrevis^p. quartl^ and 

 p. quinti dlglti. The flexor brevis digitorum of the pes has 

 one division which arises from the calcaneum and goes to the 

 second digit ; the other three heads arise from the tendons of 

 the flexor perforans. The flexor aocessorlus furnishes almost 

 the whole of the long flexor tendons of the hallux, the flexor 

 longus digitorum suppljdng the perforating tendons of the 

 second and fifth digits; while t\\e flexor hallucis longus gives 

 off the corresponding tendons of the third and fourth digits. 

 The interossei^ in the pes, appear to be represented only by 

 the pairs of muscles which act as short flexors of the basal 

 phalanges, and these lie altogether upon the plantar aspect of 

 the five metatarsal bones. The hallux has no special adductor^ 

 nor is there any transversus p)edls. In fact, the pes is almost 

 as completely a " paw " as is the manus. 



The brain has long and relatively large cerebral hemi- 

 spheres, the posterior lobes of which project far beyond the 

 cerebellum, and thus completely hide it, in the upper view of 

 the brain. The external surfaces of the hemispheres are al- 

 most smooth, but the Sylvian fissure is well marked, and there 

 is a trace of that of Rolando. On the inner face of each hemi- 

 sphere, the calcarine fissure is deep and gives rise to a well- 

 marked hippocampus minor within the posterior cornu of the 

 lateral ventricle. The corpus callosum has about a third the 

 length of the hemispheres. The septu)n lucidum is very thick, 

 and the precommissural fibres abundant. The vermis projects 

 beyond the lateral lobes of the cerebellum, and the floccidi are 

 large. 



2. The Platyrrliini are essentially quadrupedal and planti- 

 grade, though some, like the Spider Monkeys {Ateles)^ occa- 

 sionally 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 hiiid-limbs, but the reverse is the case in the Spider Mon- 



