34 NATURAL HISTORY OF VERTEBRATES. 



in which case a little space in the septum is left, known in human anatomy as the fifth 

 ventricle of the brain. It must be understood that it has really nothing to do with 

 the true ccelise, but is only a part of the great fissure which separates the two hemi- 

 spheres. The corpus callosum and fornix have important relations to the septum 

 lucidum, for the former is constituted by fibres crossing transversely between the 

 right and left hemispheres of the cerebrum, in front of and above the level of the 

 septum, while the latter arches upwards from the base of the bi'ain behind the septum, 

 and thus reaches the under surface of the corpus callosum. Concomitantly with the 

 great development of the cerebral hemispheres and their ventricles, the other parts of 

 the cceliac system become very much reduced in size. This is especially true of the 

 aula, which is very small in the higher forms. The thalamocoele is known as the 

 third ventricle, the epi- and meta-coelisB together as the fourth, while the mesocoele 

 is reduced to a mere ' iter ' connecting the two, known sometimes as the aqueduct of 

 Sylvius. The roof of the mesoccele vmdergoes a further subdivision than we find in 

 reptiles and birds, being converted into four prominences instead of two, the corpora 

 quadrigemina. 



Of the greatest importance for the functional activity of the maminalian brain is 

 the circumstance that in all except the lowest forms the surface, instead of remaining- 

 smooth, becomes much complicated by the development of fissures and intervening^ 

 convolutions. This complication of the surface has for its object the accommodation 

 of an increased amount of gray matter, for in the cerebrum, as well as in the cerebel- 

 lum, the white fibrous matter is contained in the inside, while the gray matter con- 

 taining the ganglion cells is superficial. The shape of these convolutions is very dif- 

 ferent in the cerebellum and cerebrum. In the former they are in regular foliations, 

 which determine the curious method of distribution of the white matter, known as the 

 arbor vitas, while in the latter they are far less regular, although nevertheless always 

 conforming to the same fundamental type in the same species. Of special interest 

 in connection with the form of these convolutions is the discovery within recent years 

 that particular functions are localized in parts of certain convolutions which are 

 thus regarded as the cerebral centres of these functions. 



In addition to the pia mater already referred to as carrying the vessels for the brain, 

 there is an 'arachnoid' membrane bounding a lymph space immediately outside the 

 pia, and a dura mater intimately connected with the skull, and in mammals sending 

 partitions between the lobes of the brain, which contain the great veins necessary 

 to drain away its venous blood. Sometimes the dural partitions referred to 

 ossify, as in the case of that separating the cerebrum and cerebellum of many carni- 

 vores which possess an ossified tentorium cerebelli. Before leaving the brain, a few 

 words as to the epiphysis and hypophysis cerebri (pineal or conavial and pituitary 

 bodies) will indicate the present state of our knowledge of these remarkable organs^ 

 They are apparently functionless in all living forms, but are no doubt dei-ived from 

 structures perhaps of a sensory character, which were intimately related to the brain 

 in the ancestors of the vertebrates. The epiphysis is a diverticulum of the roof of the 

 thalamocoele, while the hypophysis is develqped from the roof of the mouth, but is 

 intimately related to a diverticulum of the floor of the thalamocoele. The relations, 

 of these two diverticula suggested to Owen and others that the intestine once com- 

 municated with the upper surface of the head through this ' conario-hypophysial 

 tract,' and opened on the neural surface, as it does in arthropods. According to this 

 theory the conario-hypophysial tract is the remains of a primitive oesophagus, and the 



