X BRAIN 511 



running in the mesentery. They come into connection with numerous 

 adenoids (p. 498) in the mesentery and elsewhere, and most 

 of them communicate with a main trunk — the thoracic £/«<-/— 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. 125, 131, and 132) 

 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 applied 

 to one another along theirflat internal surfaces, and are roughly 

 conical in form, narrower in front {fronta/ Mes), broadening 

 out posteriorly {parietal lol>es) where they overlap the dien- 

 cephalon and optic lobes and abut against the cerebellum, 

 and produced downwards into the prominent temporal lobes 

 which partly overlap the crura cerebri below. Their ex- 

 ternal layer or cortex is formed of grey matter, and their 

 surfaces are smooth, except for the presence of slight fissures 

 between the lobes : in many Mammals the surface of the 

 hemispheres is highly convoluted, i.e. raised into numerous 

 winding elevations or gyri, separated by narrow grooves or 

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

 missure connecting the two hemispheres known as the 

 corpus callosum (Fig. 131 cp. c, and Fig. 132, 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, 



