NERVOUS SYSTEM 247 



From the mode of its development the central nervous 

 system is tubular ; and it retains this character throughout 

 life. The central canal is of very small size in the spinal cord, 

 but widens out considerably in the brain, its local dilatations 

 being spoken of as the ventricles of the brain. 



A. The Dorsal Surface of the Brain. 



Bemove the skin from the dorsal surface of the head. 

 Dissect away the eyelids on the right side so as to open up the 

 orbit. Slice away the roof of the cranium so as to expose the 

 brain fully, slicing through the auditory capsule, which lies 

 immediately behind the orbit, on the right si^e, but leaving 

 it uninjured on the left. 



The brain is large and nearly fills the cranial cavity : it is 

 narrow between the orbits, but expands considerably ia front 

 of them. As seen from above, the following parts are visible 

 in order from before backwards (c/. fig. 49, p. 224). 



1. The prosencephalon is the most anterior and the largest 



division of the brain. It is a smooth, somewhat 

 globular mass, lying in the expanded anterior part of 

 the cranial cavity, between the olfactory capsules and 

 the orbits. It is marked in front by a shallow median 

 groove ; and is sharply separated behind from the 

 succeeding part of the brain. It corresponds to the 

 cerebral hemispheres of higher vertebrates. 



2. The olfactory lobes are a pair of large oval masses, arising 



by stout stalks from the sides of the prosencephalon, 

 their distal surfaces being closely applied to the 

 olfactory capsules. 



3. The thalamencephalon is the narrow portion of the brain 



immediately behind the prosencephalon ; its roof is 

 very thin, so that the cavity, or third ventricle, 

 can be seen through it. From the hinder part of 

 the roof arises the pineal body, a slender tubular 

 process which extends forwards over the prosen- 

 cephalon, and ends in a slightly dilated knob attached 

 to the membranous roof of the skuU some little dis- 

 tance in front of the brain. 



