32 



NATURAL HISTORY OF VERTEBRATES. 



ventral roots corresponding to the middle three segments. On the other hand, the 

 dorsal roots of the first two segments give rise to the fifth (trigeminal) nerve, of the 

 next two to the seventh and eighth (facial and auditory), of the fifth to the glosso- 

 pharyngeal (ninth) nerve, and of the remaining four to the complex vagus (tenth) 

 nerve. That these nerves cause no marked segmentation of the adult brain is abun- 

 dantly apparent from Fig. 37, which shows how they originate in the brain of a turtle. 

 A short reference is all that our space permits to the course of the dorsal and 

 ventral branches of the dorsal roots. The former appear to be chiefly distributed to 

 the neuromastic tracts, with the exception of the fourth, which forms the auditory 

 nerve (vide supra, p. 8) while the latter course down the sides of the head, and, when 

 they arrive at the visceral clefts, fork over these, giving a slender anterior branch in 

 front of the cleft, and a stouter posterior branch behind. Intestinal branches are 

 likewise given off, which correspond to the intestinal branches of the spinal nerves 

 considered before. 



In higher animals two other nerves primitively connected with the spinal cord, the 

 spinal accessory (eleventh) and hypoglossus proper (twelfth) become associated with 

 the ten pairs of cranial nerves referred to above. 



Such a brain as that of the gar-pike forms a type from which the brains of other 

 fishes and amphibians may readily be studied. In the higher classes, however, there 



is an ever-increasing tendency towards the su- 

 premacy of two regions of the brain over the 

 others, viz., — the cerebral hemispheres and the 

 cerebellum, — parts, it will be remembered, of 

 the pros- and ep-encephalon. This tendency is 

 not very much marked in the brain of the turtle 

 (Fig. 37), but the prosencephalon even here is 

 very much more developed than in the lower 

 forms. The particular way in which this has 

 taken place can be realized by comparing Fig. 

 36, which represents a horizontal section through 

 the anterior end of the turtle's brain, with the 

 representation of a similar section through the 

 gar-pike's brain (Fig. 33). It will be observed 

 that paired outgrowths are present from the pros- 

 encephalon in the latter, but the median walls 

 and the roofs of these remain very thin, while in 

 the turtle they are nearly as thick as the rest of 

 the wall of the prosocoele. The lateral outgrowths 

 in the turtle preponderate over the central un- 

 divided part, and they grow back beyond the latter 

 so as to overlap even the mesencephalon. The 

 ventricles or cavities of these lateral outgrowths 

 remain in communication by wide ' foramina of Monro,' with the central part of the 

 prosocoele (the aula), the anterior wall of which is known as the lamina terminahs of 

 the brain. It will be observed that this lamina is of thickened nervous matter in the 

 turtle, whereas in the gar-pike it is represented only by the posterior edge of the thin 

 double septum of the prosocoele. 



The roof of the brain in the turtle is not unlike that of many lower forms, for the 



Fig. 36. — Horizontal section tiirongli anterior 

 ytaxt of brain of a turtle ; m, mesoccele ; i, 

 thalamocoele : p, aula, or axial part of pro- 

 socoele leading by the portse or foramina of 

 Monro (the arrows) into the anterior {p') and 

 posterior (p") cornua of its lateral ' ventricles ' 

 or procoeles ; ' ' 

 ccele. 



/, lamina terminalis ; r, rhino- 



