BRAIN 



205 



respects in an embryonic condition : a dorsal connection of its two 

 halves by nervous elements is very incomplete, narrow bridges of 

 nervous substance only occurring in the primitive hind-brain, the 

 posterior portion of the mid-brain, the posterior commissure, and 

 the ganglion habenulae of the right side. The main part of the 

 roof consists of membrane and vessels. 



In the larval Petromyzon or Ammoccete, very primitive condi- 

 tions are met with (Fig. 154), and, as is also the 

 case in the adult, the individual vesicles lie in 

 an almost horizontal direction one behind the 

 other ; the telencephalon consists of a median 

 part and of small paired hemispheres con- 

 tinuous anteriorly with the larger, rounded 

 olfactory lobes. The median portion of the 

 teloccele is continued transversely outwards 

 into each hemisphere, in which it gives rise 

 to a lateral ventricle : this is continued for- 

 wards for a short distance into the base of the 

 olfactory lobe, as well as backwards into the 

 hemisphere. The roof (pallium) of the median 

 portion of the ventricle is non-nervous, and 

 consists of a single layer of epithelial cells, 

 which, together with the pia mater, has been 

 removed in the preparation represented in 

 Fig. 154, A. The mid-brain and elongated 

 medulla oblongata are relatively broad, and 

 the cerebellum is represented by a mere narrow 

 ledge overhanging the fourth ventricle anteri- 

 orly. The roof of the mesocoele is formed 

 mainly by a layer of epithelial cells, and, like 

 that of the third and fourth ventricles, is 

 covered by a thickened and vascular portion 

 of the pia mater, or choroid plexus. 



The brain of Myxinoids (Fig. 155) shows 

 many peculiarities, and the morphology of its 

 parts requires further investigation. Its sub- 

 divisions are broader and more closely approxi- 

 mated than in the Lamprey, and its right 

 and left halves are more plainly marked off 

 from one another by a continuous longitudinal dorsal furrow. No 

 pallium is recognisable, at any rate in the adult. The ventricles 

 have undergone reduction, and present individual variations : in 

 the ventral region of the fore-brain there is a small isolated cavity 

 which probably represents the vestige of a third ventricle. The 

 broad olfactory lobes are separated from the telencephalon by a 

 transverse furrow. The diencephalon is not visible from the 

 dorsal side, but ventrally there is a distinct infundibular process. 

 The mid-brain is the most prominent division : the mesocoele ends 



R 



FIG. 153. DIAGRAM 

 TO ILLUSTRATE THE 

 CEREBRAL FLEX- 

 URE OF A MAMMAL. 



H H, metencephalon ; 

 Mff, mesenceph- 

 alon, which at SB 

 forms the most 

 projecting portion 

 of the brain, re- 

 presenting the so- 

 called " parietal 

 bend " ; NH, my- 

 elencephalon, form- 

 ing the " cervical 

 bend" (NB): the 

 "Varolian bend" 

 (BB) arises on the 

 ventral circumfer- 

 ence, at the junc- 

 tion between HH 

 and NH*R, spinal 

 cord ; VH, telen- 

 cephalon ; ZH, 

 diencephalon, with 

 the pituitary body 

 (H) at its base. 



