568 CLASS XVII. 



neural arches the great alse of the sphenoid {plisjilienoids Owen), 

 and for its neural spine the parietal or the two parietals. The 

 separation of the sphenoid bone into two parts is seen in the human 

 embryo (see PI. xvi. fig. 3), and it persists in most mammals, and 

 even in many monkeys, for the whole life. The posterior part 

 consists of the body of the sphenoid as far as the anterior clinoid 

 processes and the greater alaj; the anterior part consists of the 

 anterior clinoid processes and the rostrum [prespJienoid Owen) 

 with the lesser alee {orhitospheiio'ids Owen). The third vertebra 

 consists of this anterior part of the sphenoid as its centrum and 

 neural arches, whilst the frontal bone, single or in two lateral 

 halves, is its neural spine. We may remark that the posterior 

 wings of the sphenoid are with propriety termed the great wings 

 {alcB magnce) in the human cranium, but in many mammals, as the 

 ruminants and pachyderms, they are smaller than the anterior. 

 The fourth cranial vertebra has for its centrum the vomer, for its 

 neural arches the middle portion of the ethmoid (corresponding 

 to the prefrontals of fishes, which in the superior classes are com- 

 pressed and coalescent), and for its neural spine the nasal bones. 



Between the occipital bone and the second cranial vertebra the 

 temporal bone is placed; it is composed of five parts originally 

 distinct, the petrous bone {petrosal Owen^), the mastoid bone, the 

 squamous portion {squamosal Owen), that part which belongs 

 to the tympanum (the tymjyayiic bone) and the styloid process 

 {stylo-hyal). 



The internal surface of the cranium presents a cavity which, as 

 in birds, is entirely occupied by the brain, and is thus the cast of 

 its surface. The upper walls, formed by the frontal bone and the 

 parietal bones, are pretty smooth, with the exception of the impres- 

 sions of the convolutions of the brain and the more conspicuous 

 grooves caused by the vessels and the longitudinal sinus. The 



^ [The petrosal is the interpolated auditory capsule and belongs to the splanchnic 

 skeleton, it coalesces very early with the mastoid, the parapophysis of the parietal ver- 

 tebra ; the squamosal is part of the diverging appendage of the inverted arch of the 

 nasal vertebra; this last is much expanded and affords in mammals an articular sur- 

 face for the lower jaw (the frontal hjemapophysis), which in the other classes is articu- 

 lated with the pleurapophysis of its own vertebra, the tympanic ; the styloid process 

 {stylo-hyal Owen) is the pleurapophysis of the parietal vertebra. See Owen The Arche- 

 type and Homologies, &c. pp. 25 — 45.] 



