324 NERVOUS SYSTEM. 



Base of the Brain. 



The base or posterior part of the cerebrum is divided into six 

 lobes : two injerior or anterior, reposing upon the alte of the 

 ethmoid bone ; two nuddle, upon those of the sphenoid ; and two 

 superior or posterior, lodged in the fossse of tiie squamous por- 

 tions of the temporal bones. 



Over the middle lobes are two broad prominences whose sur- 

 faces are remarkably even and smooth : these are the bases of the 

 corpora striata, the parts from which the olfactory nerves are seen 

 taking their origin. 



Crura Cerebri. — Above these bodies again, in the middle of the 

 brain, are two oblong medullary eminences, intimately united 

 together: these are the crura cerebri. They rest upon the alas 

 of the sphenoid bone. They take their rise from the inferior and 

 middle lobes of the cerebrum, and are continued into an ovoid 

 protuberance above them, named the tuber annulare. Though 

 medullary without, the crus, when cut deeply into, exhibits a ci- 

 neritious hue, and the part of the interior the most darkly shaded 

 has been designated the locus uiger. 



Corpus Atbicantium. — Seated between the crura, is a small, 

 hemispherical, medullary eminence, distinguished as the corpus 

 alhicantiuui. In the human subject this body is double ; and 

 there is some appearance of a depression along its middle in the 

 horse. It is medullary without, but has a tinge of cortical matter 

 within. 



Tractus Optici. — Winding obliquely tlownward, around the 

 crura, proceeding from the terminations of the thalami, are the 

 tractus uptici: they are to be regarded as the roots of the optic 

 nerves, with which they are continuous. They are entirely me- 

 dullary. 



Crura Cerebelli. — Higher up and more outward than the crura 

 cerebri, are the crura cerebel/i— two stout, cylindroid, medullary 

 chords joining the lateral lobes of the cerebellum to the tuber 

 annulare. These parts consist of a prolongation of that portion 

 of medullary substance which forms the trunks of the arbor vitae. 



The tuber annulare vel pons Varolii (whose situation now 

 need not be pointed out) may be said to be constituted of the 

 united substances of the crura cerebri and crura cerebelli : it is, 

 in fact, the common termination or medium of junction of the 

 cerebrum with the cerebellum. It is supported by the cuneiform 

 process of the posterior occipital bone. Its surface is marked by 

 a shallow depression along its middle, which gives it the appear- 

 ance of having lateral eminences. Internally, the tuber exhibits 

 a mixture of cortical and medullary matter. 



