2 20 VERTEBRATE SKELETON 



in an occiput, either strongly inclined backwards or even all but horizontal. 

 In the young the cranium is rounded, permanently so in man, but other genera 

 tend towards a prognathism in the adult and a consequent elongation of the 

 skull. .\ sagittal crest is common (except in American monkeys), especially so 

 in Anthropomorphas (tig. 231, B), and the old world species often have a strong 

 supraorbital crest. All Primates have pneumatic bones. The orbits are 

 directed forwards and almost completely separated from the temporal fossae. 

 Their position results in a narrowing of the posterior part of the nasal cavities 

 and reduction of the conchae. The lateral wall of the orbit in American species 

 is largely formed by the zygomatic bone; in old world genera more by alisphenoid 

 and frontal. In all, the ethmoid enters its median wall and the lacrimal bone 

 and its foramen are within the orbit. Fusion of the frontals is common and 

 the nasals may ankylose. The latter bones and the premaxilLTC usually 

 bound the nares; the premaxillae fuse early with the maxillas. 



The alisphenoid has a large pterygoid process, separated from the true 

 pterygoid by an entopterygoid fossa of varying width, and is pierced by the 

 foramen rotundum. It is stated that young apes have a thin interorbital 

 septum; whether this be an inheritance from some tropibasic ancestor or a new- 

 acquisition is not decided. Petrosal and mastoid fuse; the tympanic is annular, 

 walling the external meatus and usually fusing with the petrosal and squamosal, 

 forming the temporal bone. The shallow mandibular fossa has a postglenoid 

 process. The halves of the lower jaw are fused, the ramus high, its angle 

 rounded, the coronoid process well developed and the condyle is broadly 

 transverse. 



APPENDICULAR SKELETON 



Vertebrates may have two kinds of appendages, median fazygos 

 or unpaired) and paired. The discussion of their origin (p. 241) best 

 follows the account of the structure of both. Both are integumental 

 folds into which mesenchyme and usually mesothehal muscles 

 extend. Development shows that both kinds are markedly meta- 

 meric at first, the repetition of parts affecting not only the primitive 

 musculature, but nerves, blood vessels and skeletal parts as well. 



In their simplest form the skeletal parts of the appendages con- 

 sist of a series of parallel cartilage rods or rays, to which, in fishes are 

 added more distal parts (actinotrichia) , horny in character, which are 

 not metameric, but are paired in origin and more numerous than 

 the somites entering the appendage. Primitively each cartilage ray 

 is divided into a basale, either enveloped in the body or close to it, 

 and one or more radialia which extend into the free appendage. In 

 most Vertebrates these parts are considerably modified, least so in 

 the median appendages. 



