THE PIGEON. 193 



5 2. The character of the cranial bones as seen in a longi- 

 tudinal section of the skull : they consist of a dense but 

 very thin outer layer or external table, of a similar inner 

 layer or internal table, and of an intermediate spongy 

 substance or diploe : the latter is especially thick in the 

 anterior part of the frontals and in the basi- and para- 

 sphenoid. 



53. The hollows on the inner surface of the brain-case 

 for the lodgment of various parts of the brain : the basis 

 cranii is deeply concave from before backwards for the re- 

 ception of the medulla oblongata (Fig. 58, m.o) : in the 

 frontal region, or dorsal portion of the brain cavity, is the 

 large cerebral fossa for the reception of the cerebral 

 hemisphere (Fig. 58, c.li) ; below this and separated from it 

 by a well marked horizontal ridge of bone is the rounded 

 optic fossa for the optic lobe (Fig. 58, o.l) ; behind and 

 below these two hollows is the cerebellar fossa for the 

 cerebellum (Fig. 58, cb), limited in front by a ridge the 

 tentorial ridge which passes obliquely upwards and 

 backwards from the basis cranii to meet its fellow of the 

 opposite side in the roof of the skull somewhat anterior to 

 the lambdoidal ridge. 



54. The auditory capsule, a considerable thickening 

 of the brain-case just external to the foramen magnum, and 

 encroaching upon the optic and cerebellar fossae : it is 

 limited above by a well marked semicircular ridge, which 

 indicates the position of the anterior semicircular 

 canal, a swelling at its anterior end marking the position of 

 the ampulla of that canal (see figure of cod's auditory 

 organ, p. 123): within the arch of this canal is another 

 ridge, indicating the inner (anterior) portion of the pos- 

 terior semicircular canal. 



The position and relations of the three semicircular canals are best 



O 



