100 ZOOLOGY SECT. 



afferent and efferent fibres, the afferent derived from the dorsal, 

 the efferent from the ventral roots of the spinal nerves, and both 

 traceable, through those roots, into the grey matter of the cord. 

 The sympathetic nerves supply the enteric canal and its glands, 

 the heart, blood-vessels, &c., and are therefore denominated 

 splanchnic nerves. 



As already mentioned, the anterior end of the nervous system 

 undergoes, at a very early period, a marked dilatation, and is 

 distinguished as the hrain (Fig. 778). Constrictions appear in the 

 dilated part and divide it into three bulb-like swellings or vesi- 

 cles, the for e-h^ain( A, f. b.), mid-brain (m. b.) and hind-brain {h. b.). 

 Soon a hollow outpushing grows forwards from the first vesicle 

 (B, prsen), and the third gives off a similar hollow outgrowth 

 (cblm.) from its dorsal surface. The brain now consists of five 

 divisions : the prosencephalon (prs. en.) and the dieneephalon (dien.), 

 derived from the fore-brain : the mid-brain or mesencephalon 

 (m. b.) which remains unaltered : the epencephalon or cerebellum 

 (ebbn.), and the metencephalon or medulla oblongata (med. obi.), 

 derived from the hind-brain.^ Additional constrictions appear in 

 the medulla oblongata giving it a segmented appearance, but they 

 disappear as development proceeds, and, whatever may be their 

 significance, have nothing to do with the main divisions of the 

 adult organ. The original cavity of the brain becomes corre- 

 spondingly divided into a series of chambers or ventricles, all 

 communicating with one another and called respectively the fore- 

 ventricle or prosoeaele, third ventricle or diaccele, mid-ventricle or 

 mesoccde, cerebellar ventricle or epiccele, and fourth ventricle or 

 metacode. 



In some Fishes the brain consists throughout life of these five 

 divisions only, but in most cases the prosencephalon grows out 

 into paired lobes, the right and left cerebral hemispheres or 

 parencephala (I-L, c./i.), each containing a cavity, the lateral 

 ventricle or paracoele {fa. ca.) which communicates with the 

 diaccele {di. cm!) by a narrow passage, the foramen of Monro (/. m.). 

 Moreover, each hemisphere gives off a forward prolongation, the 

 olfactory bidb or rhinencephalon (olf. I.), containing an olfactm'y 

 ventricle or rhinocoele (rh. cm.) : when there is an undivided prosen- 

 cephalon, the olfactory bulbs (C, D, olf. I.) spring from it. In the 

 embryo of some forms there is a median unpaired olfactory 

 bulb, like that of Amphioxus. The part of the cerebral hemisphere 

 with which the olfactory bulb is immediately related is the 

 olfactory lobe. 



The brain undergoes further complications by the unequal 

 thickening of its walls. In the medulla oblongata the floor becomes 

 greatly thickened (D, H, K), while the roof remains thin, con- 



^ The prosencephalon is sometimes called the telencephalon, the epencephalon 

 the metencephalon, and the metencephalon the myelencephalon. 



