450 MR HAROLD AXEL HAIG ON 



(iv.) The peculiar shape of the urinary bladder. According to HEPBURN,* the 

 bladder in the adult seal is represented by the whole length of the foetal 

 organ, since no shrinkage takes place in the cephalic portion to form a 

 definite urachus. 



B. General Topographical Anatomy of the Head. 



I. The oral, nasal, and cranial cavities, with their contents (PL II.). 



A median sagittal section of the head was made, so that the oral, nasal, and 

 cranial cavities were laid bare ; whilst, in order to expose the contents of the nasal 

 fossae, the nasal septum was subsequently removed. 



Rudiments of the incisor milk-teeth were cut through in the upper and lower 

 jaws, and the surface of the mesial section of the hypophysis cerebri exposed, the 

 hypophysis lying in the sella turcica of the developing sphenoid bone at the base of 

 the skull (PL II. fig. 3 3 ). 



Other points worthy of note in such a hemisection of the head are : 



(a) The elongated tongue (PL II. fig. 3 2 ). 



(b) The falx cerebri, covering the mesial surface of the right cerebral hemisphere 



(PL II. fig. 3 9 ) ; about the middle of the concave edge of the falx the 

 corpus callosum is seen in section (PL II. fig. 3 9 '.) 



(c) The optic thalamus (PL II. fig. 3 8 ) continued posteriorly into the 



mesencephalon and pons ; dorsal to these latter, the Sylvian aqueduct with 

 its roof, in which the rudiments of the corpora quadrigemiua (fig. 3 7 ) 

 are to be made out ; posterior to these are seen the mesial section of the 

 cerebellar vermis, and, ventral to this, the 4th ventricle and medulla 

 oblongata (PL II. fig. 3 3 ' 6 ' 5 - 4 ). 



(d) The cavity of the 3rd ventricle, with its extension into the infundibulum of 



the hypophysis. 



After the removal of the falx cerebri and nasal septum, the following additional 

 structures come into view : 



In the cranial cavity (PL II. fig. 5) : 



(e) The mesial surface of the cerebral hemisphere (/;,.). 



(/) The olfactory lobe (olf. I.), lying between the fore-brain and the anterior 

 boundary of the cranial cavity : the lobe is not a large one, and does not 

 appear to give off many nerve-filaments to the ethmoidal region of the 

 nasal fossa. 



In the region of the nasal fossae : 



(g] The rudiment of the ethmo-turbinal bone (eth. t.) lying at the superior angle ; 

 this rudiment, which at the present stage is cartilaginous, shows three ridges 



* Tram. Roy. Soc. Edin., vol. xlviii. part i., No. 3, 1913. 

 (ROT. soc. EDIN. TRANS., VOL. L., 228.) 



