LEPUS 155 



study, but the transverse tracts or commissures are easily seen 

 in a longitudinal section of the brain. Of these, the corpus 

 callosum (connecting cerebral cortex) and the pons Varolii 

 (connecting cerebellar cortex) have already been mentioned. 

 In addition there are the following : the hippocampal com- 

 missure, which connects the two hippocampal lobes, running 

 ventral and posterior to the corpus callosum and dorsal to the 

 3rd ventricle ; the anterior commissure, connecting the two 

 halves of the corpus striatum, and running in the anterior wall 

 of the 3rd ventricle or lamina terminalis ; the habenular 

 commissure, connecting the optic thalami, running across the 

 roof of the 3rd ventricle just beneath the pineal body ; the 

 posterior commissure, in the roof of the midbrain ; the inferior 

 commissure, crossing the floor of the 3rd ventricle close to the 

 optic chiasma. The " soft commissure " is not really a com- 

 missure, since it does not transmit a transverse tract of fibres. 



Meningeal Membranes. — The brain is surrounded by the 

 vascular pia mater, which projects into the lateral ventricles, 

 the 3rd and the 4th ventricles, forming in each a choroid plexus. 

 Outside the pia mater is the arachnoid membrane, and outside 

 this again is the protective and hard dura mater. The cerebro- 

 spinal fluid which fills the canal of the spinal cord and the 

 ventricles of the brain communicates with the space contained 

 by these meningeal membranes through an opening in the roof 

 of the 4th ventricle, the foramen of Magendie. 



Nerves. — The distribution of the peripheral nerves in the 

 head is not dissimilar from that in lower forms, but attention 

 may be paid to the conditions in the region of the neck. On 

 each side of the neck, just lateral to the trachea, there are a 

 number of nerves running parallel with the carotid artery and 

 jugular vein. The vagus is one of these : it emerges from the 

 skull (through the foramen lacerum posterius) and swells into a 

 ganglion from which a nerve runs backwards. It soon gives off 

 an anterior laryngeal nerve which runs to the larynx, and a small 

 depressor nerve which accompanies the vagus in its course 

 backwards to the heart. The vagus passes ventral to the aortic 

 arch on the left, and ventral to the innominate artery (which 

 corresponds to the aortic arch) on the right. Immediately after 



