654 



THE NERVOUS SYSTEM. 



The remaining structures which contribute to the formation of the limbic lobe 

 will be subsequently described. 



The olfactory lobe is situated on the under surface of the frontal lobe. It is 

 rudimentary in man and some other mammals, but in vertebrates generally it is 

 well developed, and consists of a distinct extension of the cerebral hemisphere, 

 enclosing a portion of the anterior horn of the lateral ventricle. In man it is long 

 and slender, and may be described as consisting of two parts, the anterior and 

 posterior olfactory lobules. 



The anterior olfactory lobule is made up of: (1) the olfactory bulb ; (2) the 

 olfactory tract; (3) the trigonum olfactorium ; and (4) the area of Broca. 



Frontal lobe. 



Posterior 

 division 

 of Sylvian 

 fissure. 



Temporo- 



sphenoidal 



lobe. 



Occipital 

 lobe. 



FIG. 350. Base of the brain. 



(1) The olfactory bulb is an oval mass of a reddish-gray color, which rests on 

 the cribriform plate of the ethmoid bone, and forms the anterior expanded extrem- 

 ity of the olfactory tract. Its under surface receives the olfactory nerves, which 

 pass upward through the cribriform plate from the olfactory region of the nose. 

 Its minute structure will be subsequently described. 



(2) The olfactory tract is a band of white matter, triangular on section, the 

 apex being directed upward. It lies in the olfactory sulcus on the under surface 

 of the frontal lobe. Traced backward, it is seen to divide into two roots, an outer 

 and an inner. The outer root passes across the outer part of the anterior perforated 

 space to the nucleus amygdalae and the anterior part of the gyrus hippocampi. 

 The inner root turns sharply inward, and ends partly in Broca's area and partly in 



