210 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



cord, contained in the neural canal. They are invested 'by a 

 delicate pigmented vascular membrane, the pia mater, and the 

 neural canal is lined by a firm fibrous membrane, the dura mater, 

 Between the two is a lymphatic arachnoid space, the walls of 

 which are formed by a delicate arachnoid membrane. 



The brain is the anterior enlargement of the cerebro-spinal 

 axis, contained in the cavity of the brain-case. At an early stage 

 of development it consists of three vesicles, anterior, middle, and 

 posterior. The adult parts derived from these constitute the 

 fore-, mid-, and hind-brains. That part of the fore-brain which 

 corresponds to the original anterior vesicle is known as the 

 thalamencephalon, and contains a vertical slit-like cavity, the 

 third ventricle, the anterior boundry of which is a thin lamella of 

 brain-substance, the lamina terminalis. The thin roof of the 

 third ventricle, is covered by a vascular membrane, the choroid 

 plexus, and in the young tadpole is connected with a small 

 rounded body, the pineal "gland," by a hollow pineal stalk, 

 directed upwards and backwards. The formation of the roof of 

 the skull pinches off the pineal gland, which in the adult 

 underlies the skin covering the top of the head. The floor of 

 the third ventricle is produced downwards in its hinder part into 

 a funnel-like projection, the infundibulum, with which a flattened 

 sac, the pituitary body, is connected. The much-thickened side- 

 walls (optic thalami) are connected together behind by a small 

 band, the posterior commissure. A similar anterior commissure 

 runs transversely across through the substance of the lamina 

 terminalis, and connects the corpora striata (see below). 



The rest of the fore-brain is made up of the structures deve- 

 loped from a pair of lateral outgrowths of the anterior cerebral 

 vesicle. These are (1) the cerebral hemispheres continuous with 

 the antero-lateral parts of the thalamencephalon, which structure 

 they partly overlap, and in front of which they extend; and 

 (2) the olfactory lobes into which the hemispheres pass anteriorly. 



The cerebral hemispheres are smooth, ovoid bodies, broadest 

 behind, and closely approximated in the middle line. Each 

 contains a lateral ventricle, the inner wall of which presents an 

 elevation, the corpus striatum, and which communicates with the 

 third ventricle behind by a small aperture, the foramen of Monro. 

 The olfactory lobes are two small bodies in front of and con- 

 tinuous with the cerebral hemispheres, from which they are not 



