804 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



F. 



elevations (gyri) and depressions (sulci), convolutions and Assures, form 



so as to give to the brain its characteristic 

 appearance, the degree of external com- 

 plexity differing in different classes of 

 animals. 



In fact, the strongest point in favor 

 of the high importance of the cerebrum, 

 and especially its connection with the 

 mental functions, is seen in the progressive 

 complexity of its surface in passing from 

 the lower to the higher animals. 



In lishes, amphibia, and reptiles the 

 cerebral cortex is smooth, and but a faint 

 trace of the formation of fissures is to 

 be seen in birds. In the lowest mammals 

 also the hemispheres are smooth, as in the 

 marsupials, the lowest rodents, if not also 

 in the lowest so-called quadrumana, as in 

 the lemurs. But, ascending to the higher 

 orders of mammals, the hemispheres be- 

 come more and more sulcated on the 

 surface, until the ridges or convolutions 

 become more and more numerous and 

 complex as we reach the highest mammals 

 or the highest genera in the several orders. 



The cerebral convolutions may be 

 said to be characteristic of the brains of 

 mammals, and may be considered, firstly, in regard to their general 

 plan, and, secondly, their relative complexity within that plan. 



The highest degree of complexity of the cerebral convolutions is seen in the 

 brain of man, of which the most important are represented in the following 

 diagrams (Figs. 347, 348, 349). Each cerebral hemisphere is subdivided on its 

 external surface into five lobes — the frontal, parietal, occipital, temporo-sphenoidal, 

 and island of Reil. 



In the (1) frontal lobe are found three convolutions — the superior, central. 

 and inferior frontal convolutions. Behind these comes the ascending fronted, sepa- 

 rated from them by the precentral fissure and bounded posteriorly by the fissure of 

 Rolando, which forms the anterior boundary of (2) the parietal lobe, the latter being 

 limited below by the fissure of Sylvius and behind by the parieto-occipital fissure. 

 In this lobe are found the ascending parietal convolutions immediately behind the 

 fissure of Rolando, supra-marginal convolution arching around the posterior 

 extremity of the fissure of Sylvius, and the angular gyrus arching around the end 

 of the first temporo-sphenoidal fissure. 



(3) The temporo-sphenoidal lobe, bounded in front by the fissure of Sylvius, 

 contains three horizontal convolutions, superior, middle, and inferior temporo- 

 sphenoidal convolutions, the first two being separated by the parallel sulcus. 



(4) The occipital lobe is separated from the parietal lobe by the parieto- 

 occipital fissure, and likewise contains three convolutions on its outer surface, — the 

 superior, middle, and inferior occipital convolutions. 



Fig. 346.— Diagram of a Hor- 

 izontal Section of a 

 Vertebrate Brain, after 

 Huxley. (Yeo.) 

 01/, Olfactory lobes ; L.t, lamina termi- 

 nalis ; C.S, corpus striatum ; Th, optic thal- 

 amus ; Pa, pineal gland ; Mh. mid-brain : 

 Cb, cerebellum; M.O, medulla oblongata; 

 1, olfactory ventricle ; 2, lateral ventricle ; 

 3, third ventricle ; It, fourth ventricle : + 

 itar e tertio ad qnartum ventriculum ; F.iL, 

 foramen of Monro ; H, optic nerves. 



