ORIGIN OF THE ENCEPHALIC NERVES. 589 



closed. Some medullary fibres have been traced from it to the 

 optic nerve. It is closely embraced by the pia mater. ^ 



Between the eminentia mammillares, in front, the pons Varo- 

 lii, behind, and the crura cerebri on the sides, is found a layer 

 of whitish gray matter, called the pons Tarini, or locus perfora- 

 tus. It forms a bridge between the parts above mentioned, so 

 as to aid in closing the bottom of the third ventricle, and is 

 perforated by several thick tufts of arteries which are distributed 

 in the optic thalami. 



The Pituitary gland or hypophysis cerebri, is a small body 

 weighing about ten grains, placed in the cavity of the sella 

 turcica of the sphenoid bone, where it is closely embraced 

 upon its side and part of its upper surface, by the processes 

 of the dura mater forming the cavernous sinus, which mem- 

 brane is reflected into the cavity of the sella turcica, and forms 

 a capsule to the gland. 



It consists evidently of two lobes, an anterior and posterior, 

 as may be seen in the human subject. It is large, and may 

 be studied to advantage in the calf, where it forms a body 

 about an inch long, and five-eighths of an inch wide. The 

 posterior lobe is about a third the size of the anterior, and 

 receives the apex of the infundibulum, through an oval opening 

 left in the dura mater. It is conoidal in front, and received in a 

 corresponding cavity of the anterior lobe, between which is 

 reflected a partition of very vascular cellular tissue from the side 

 of the capsule. The large lobe is cineritious and granular in its 

 centre, and medullary on its outer face. 



The posterior is of a grayish-white color, and presents two 

 eminences; one, which is continued forwards to the cineritious,. 

 centre of the anterior lobe, and one, which in the calf is evi- 

 dently fibrous and continuous backwards with the infundibulum. 

 The use of this organ is not understood ; in the human foetus, it 

 is larger at the fifth month of intra-uterine life than at birth. It 



O 



is proportionably larger in the inferior animals than in man, and 

 in fishes it forms a large lobe with a cavity in its centre. Wenzel 

 asserts that he has frequently found it diseased in epileptic sub- 

 jects ; and latterly Arnold has traced to it, several branches of 

 the sympathetic nerve which seem to originate from it. 

 VOL. ii. 50 



