102 



ANATOMY OF VERTEBRATES. 



Such are the essential characters of the Mammalian ( prosen- 

 cephalon.' 



70 



Lateral ventricle, Echidna. 



The chief modifications of the Mammalian brain, as 

 above characterised, will next be noticed in the 

 different leading groups of the class. 



A. Lyencephala. In the Ornithorhynchus, 

 the brain, figs. 52 and 69, is to the weight of the 

 body as 1 to 130 ; the hemispheres are triangu- 

 lar, depressed, the broader posterior part over- 

 lapping the optic lobes, and reaching to the 

 cerebellum. With the exception of the hippo- 

 campal fissure, fig. 69, 4, and the depression 

 lodging the rhinencephalic cms, the surface is 

 unbroken or smooth, with a few vascular im- 

 pressions diverging from the fore part. The 

 medulla oblongata is broad and depressed ; the 

 corpora pyramidalia, fig. 51, a, are in very low relief; the corpora 

 olivaria, a! ', expand as they advance ; they are crossed anteriorly 

 by the ( corpora trapezoidea,' b, which are large ; the ( pons,' c 9 is 

 narrow : anterior to it is a large ganglionic body, c', from which 

 issues the huge trigeminal nerve, 5. The longitudinal groove be- 

 tween the optic lobes is shallow ; 

 it is wanting in the small and 

 low i testes.' The hippocampus 

 is the chief prominence within 

 the ventricle of the hemisphere ; 

 the corpus striatum is long and 

 narrow. 



The brain of the Echidna, 

 fig. 71, is relatively larger than 

 in the Ornithorhynclius, and the 

 exposed outer surface of the 

 hemispheres is extended by con- 

 volutions. The cerebral hemi- 

 spheric cavity is mainly occupied 

 in both Monotremes by the ' hip- 

 pocamp,' fig. 70, h, which con- 

 stitutes a great part of its floor 

 as well as inner wall. This, with much of the hippocamp, is 

 removed in fig. 71, to show the proportions of the ' corpus stria- 

 tum,' s, and to bring into view the thalami, t : these are divided 

 from the s nates,' r, by a linear groove ; the ( testes,' s, are half 

 the size of the f nates,' and the median longitudinal groove, 

 which is shallow between the nates, is not continued further 



Drain and lateral ventricle, hippocampus removed 

 Echidna. 



