528 THE RABBIT CHAP. 



running in the mesentery. They come into connection with numerous 

 adenoids (p. 515) in the mesentery and elsewhere, and most 

 of them communicate with a main trunk the thoracic duct which 

 extends from the abdomen through the thorax on the left and upper 

 side of the aorta. The thoracic duct also receives the lymphatics from 

 the right side of the head and neck and the fore-limb, and opens into 

 the veins at the junction of the left external jugular and subclavian : the 

 lymphatics of the right side of the head and neck and right fore-limb 

 communicate with the corresponding veins of the right side. 



Nervous System. The brain (Figs. 135, 141, and 142) 

 reaches a much higher development than in the other Verte- 

 brates we have already studied. The prosencephalon is sub- 

 divided into two cerebral hemispheres (ch\ of much larger 

 relative size than those of the frog (Fig. 49) and forming 

 about two-thirds of the whole brain. They are closely ap- 

 plied to one another along their flat internal surfaces, are 

 roughly conical in form, narrower in front (frontal lobes], 

 broadening out posteriorly (parietal lobes) where they over- 

 lap the diencephalon and optic lobes and abut against the 

 cerebellum, and produced ddwnwards into the prominent 

 temporal lobes which partly overlap the crura cerebri below. 

 Their external layer or cortex is formed of grey matter, and 

 their surface is smooth, except for the presence of slight 

 fissures between the lobes : in many Mammals the hemi- 

 spheres are highly convoluted, i.e. raised into numerous 

 winding elevations or gvri, separated by narrow grooves or 

 sulci. A broad transverse band of nerve-fibres forms a 

 commissure connecting the two hemispheres known as the 

 corpus callosum (Fig. 141 cp. <r, and Fig. 142, cp. d) : this 

 structure is confined to the Mammalia, and is even wanting 

 in certain of the lower members of the class. The olfactory 

 lobes (olf) are club-shaped, and extend backwards along the 

 ventral surface of the hemispheres in the form of narrow 

 bands as far as the temporal lobes. 



