74 WILLIAM PATTEN. 



To sum up what has preceded, we find tliat in the brain of 

 Limulus there is an actual or potential agreement throughout 

 with the Vertebrate brain. There is (fig. 43) the folding of 

 the anterior end of the cerebral vesicle downwards and back- 

 wards to form the infundibulum ; the upward growth of the 

 co-ordinating centres, or the cerebral hemispheres, and their 

 expansion in all directions at the summit to cover the rest of 

 the brain. There are the lateral ventricles communicating in 

 front with each other, with the cavity in the epiphysis, or 

 median eye tube, and with that of the olfactory lobes, below 

 with the infundibulum, and backwards and downwards with 

 the "iter^^ and the third ventricle. Its roof is a thin mem- 

 brane or pallium ; its floor the three great lobes of the cerebral 

 hemisphere — the posterior, the anterior lateral, and the median 

 internal or corpus striatum. 



There is the great cavity in the centre of the brain corre- 

 sponding to the imperfectly separated third and fifth ventricles 

 and the " iter." It is bounded in front by a thin membrane, 

 'Mamina terminalis " (fig. 43, l.t.), above by the corpora 

 striata (c. 5.), and on the sides by the cerebral peduncles and 

 optic thalami, and on the floor by the middle and inferior 

 commissures. It communicates anteriorly and above with 

 the lateral ventricles, behind with the unroofed " iter," and on 

 the sides with a cavity leading to the roots of the lateral eye 

 nerves. 



There are three commissures to the brain — one the inferior 

 commissure in the roof of the infundibulum {a. com.), the very 

 large middle commissure in the floor of the third ventricle [m. 

 com.), and the posterior commissure in what would be the roof 

 of the mid- brain, just behind the posterior end of the cerebral 

 hemispheres [p. com.). 



There is the fourth ventricle, a large rhomboidal space 

 imperfectly covered by the medullary folds (fig. 48). Its floor 

 is composed of the united cross-commissures of the thoracic 

 neuromeres in the position of the future " pons," and its sides 

 are formed by the great diverging masses of nerve-substance, or 

 crura, leading up to the fore-brain. 



