FUNCTIONS OF THE BEAIN. 



803 



tract descending in the anterior column may either supply the muscles 

 which always act together on both sides of the body ; or, according to 

 other observers, they cross in the anterior commissure to join the lateral 

 pyramidal tract (Fig. 343). 



The paths of conduction of motion and sensation will again receive 

 attention when the upward connections of the different columns of the 

 cord have been traced. 



VI. THE FUNCTIONS OF THE BEAIN. 

 In its earliest stage of development the brain simply consists of the 

 dilated extremity of the medullary tube formed by the turning in of the 

 medullary folds of the ectoderm. Almost immediately after the closure 

 of the medullary canal its anterior termination is seen to become differ- 

 entiated into three bilateral sj'mmetrical vesicular dilatations, which are 

 1^ the starting points of the fore-, mid-, and hind- 



brain. From the fore-brain the cerebral lobes 



Fig. 344.— Diagram of 

 the Cerebral Ves- 

 icles of the Brain 

 of a Chick at the 

 Second Day of In- 

 cubation, after Ca- 

 diat. ( Yeo.) 



1, 2, 3, cerebral vesicles ; 0, 

 ' optic vesicles. 



Fig. 345.— Diagram of a Vertical Longitxtdinal Section 

 of a Developing Brain of a Vertebrate Animal, 

 showing the Relations of the Three Cerebral 

 Vesicles to the Different Pakts of the Adult 

 Brain, after Huxley. (Yeo.) 



01/, olfactory lobefl; F.M, the foramen of Monro; C.S, corpus striatnm; Tb, 

 optic thalamus; Pii, pineal glands; M b, mid-brain ; Gb, cerebellum; 31.0, medulla 

 oblongata; Ifmp, cerebral hemispheres; ThE, thalamencephalon ; Pi/, pituitary 

 body; C.Q, corpora quadrigemina; O.C. crura cerebri ; P. V, pons varolii; I-XII, 

 regions from which spring the cranial nerves; 1, olfactory ventricle; 2, lateral ven- 

 tricle; 3, third ventricle; It, fourth ventricle. 



develop as two hemispherical vesicles (prosencephalon), which, in the 

 brain of the highest mammals, so increase in size upward and backward 

 as to cover more or less completely the remainder of the primary cere- 

 bral vesicle, the mid-, and hind- brain, so that the mid-brain, composed 

 of the optic thalamus, corpora quadrigemina, and corpora striata 

 {mesencephalon), lies beneath the hemispheres. From the hind-brain 

 originate the cerebellum, pons varolii, and the medulla oblongata 

 (myelencephalon). At first all these parts consist of thin-walled vesicles 

 communicating with each other and with the interior of the central 

 canal- of the spinal cord (Figs. 344, 345 and 346). 



In higher stages of development the walls of these cerebral vesicles 

 become not only thicker and their cavities smaller and smaller, but 



