528 THE RABBIT CHAP. 



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 abdprnen 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 

 Vertebrates we have already studied. The prosence- 

 phalon is subdivided into two cerebral hemispheres (c.h), 

 of much larger relative size than those of the frog 

 (Fig. 49) and forming about two-thirds of the whole brain. 

 They are closely applied 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 overlap the diencephalon 

 and optic lobes and abut against the cerebellum, and 

 produced downwards 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 convex and smooth, except for the presence of 

 slight lateral grooves between the lobes : in many 

 Mammals the hemispheres are highly convoluted, i.e., 

 raised into numerous winding elevations orgyri, 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 (Figs. 141 and 

 142, cp. cl) : 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. 



