144 ANIMAL BIOLOGY. [Part I. 



the brain-case always have a definite relation to the nerves which 

 pass out. The orbito-sphenoid lies between the olfactory and 

 sphenoidal, and contains the optic foramina; the ali-sphenoid 

 between the sphenoidal and f. 1. medium, being perforated by 

 the rotundum and ovale where they exist ; the periotic between 

 the f. 1. medium and the f . 1. posterius ; the ex-occipital behind 

 the f. 1. posterius, and perforated by the condylar. 



The inner surface of the brain-case is divided into three 

 cranial fossae : (1) the olfactory fossa, from the cribriform to an 

 ill-marked ridge on the frontal ; (2) the cerebral fossa, from this 

 ridge to the tentorial plane, which is marked by a ridge along 

 the periotic, and by the line of junction of the parietal and 

 supra-occipital (the tentorium, which is membranous in the 

 rabbit, is ossified in the dog or cat) ; (3) the cerebellar fossa, 

 behind the tentorial plane. The plane of the cribriform is 

 called the ethmoidal plane, that of the occipital bone the 

 occipital plane. In comparative mammalian anatomy, the 

 angles made with each other by the ethmoidal, tentorial, and 

 occipital planes are of importance. The student should, if 

 possible, compare them in the hedgehog, the rabbit, the dog, 

 and man. 



In the basi-sphenoid is a depression, the sella turcica (44, A., 

 s.t.), for the lodgment of the pituitary body. It is bounded 

 before and behind by anterior and posterior clinoid processes. 



On the external surface there are sutures marking the contact 

 of separable bones. (1) The frontal suture, between the two 

 frontals ; (2) the inter-parietal or sagittal suture, between the two 

 parietals ; (3) the fr onto -parietal or coronal suture, between the 

 frontals and parietals ; (4) the occipito-parietal or lambdoidal 

 suture, between the occipital and parietals. 



(2.) The Olfactory Chamber. This lies in front of the brain- 

 case. It is bounded posteriorly by the cribriform plate, and is 

 divided into two compartments by the median lamina perpendicu- 

 laris, which is continued forwards by the cartilaginous septum 

 nasi. The lamina perpendicularis and the cribriform plate form 

 parts of one bone, the mesethmoid. Beneath the cartilaginous 

 septum lie the coalesced vomers, forming a trough. The roof of 



