464 



COMPARATIVE ANATOMY. 



fenestra rotunda as well as a fenestra ovalis. The posterior portion 

 of the petrosal, which is likewise ossified from an independent centre, 

 is attached to the sides of the exoccipitals, and is distinguished as 

 the mastoid portion, in consequence of its carrying the mastoid 

 process in Man. Superiorly, the squamosal (Sq) is attached to the 

 petrosal, and is sometimes fused with it to form the temporal bone, 

 of which it then forms the "squamous portion." In some forms it 

 is completely shut out from the cranial cavity; in others a small 

 portion only can be seen from the inner surface of the skull (Cetacea, 

 Ruminants). It is in the Primates only that this portion is of any 

 great size, and this leads to the well-known arrangement which is 

 seen in Man. A process (zygomatic process) of the squamosal which 

 is directed forwards, takes part in the formation of the jugal arch. 



In front of the temporal region is the sphenoidal region, which 

 is made up of two perfectly developed segments. The basal 



piece of the hinder 

 one (basi-sphenoid) 

 (Fig. 253, Sj>h) abuts 

 directly on the basi- 

 occipital, and carries 

 at its sides the alao 

 tempdrales (ali- sphe- 

 noids). Thepresphe- 

 noid with its alae orbi- 

 tales (orbito-sphenoid) 

 (Ps) lies in front of 

 the basi - sphenoid. 

 The two median pieces 

 are separate through- 

 out life, or for a long- 

 time, in Mammals. 

 In Man they fuse very 

 early to form the so- 

 called body of the 

 sphenoid. 



In the roof of the skull we again find the well-known covering 

 pieces, which increase in extent as the cranial cavity grows larger. 

 The parietals (Figs. 252, 253, Pa) are often fused together (in the 

 Monotremata, many Marsupialia, the Ruminantia, and Solidungula). 

 Between them, a special bone, which marks off the supra- occipital, 

 projects from behind forwards; this interparietal is generally, as 

 in the Primates, fused with the supra-occipital (Figs. 252, 253, Jp), 

 but, at times, with the parietals (Rodentia and Ruminantia). 



The frontals (Fr) are attached to the ala3 orbitales; they are 

 always paired ; they are fused in some, as in Elephas, Rhinoceros, 

 and also in the Prosimia), Insectivora, Chiroptera, and Primates. 



The most anterior segment of the primordial cranium presents 

 the most important modifications. It is developed into the wall of 

 the nasal cavity owing to the formation of various laterally and 



Fig. 253. Vertical median section through the same 

 skull. Ob Basi-occipital. Ps Presphenoid. Eth Eth- 

 moid (vertical plate of the cribriform bone, the anterior 

 edge of which is continued into the cartilaginous par- 

 tittons between the nares, which is not seen in this 

 figure). Eth Turbinate bones. Vo Vomer. s/Frontal 

 sinus. The other letters as in the preceding figure. 



