THE BRAIN 



85 



Most of the portions of the brain referred to in the foregoing 

 paragraphs are not units homogeneous in respect of structure or 

 function but are made up of constituents of varied significance. 

 An attempt to indicate briefly the functions of various parts has 

 been made in connection with the somewhat more detailed descrip- 

 tion in the directions for dissection (pp. 348-366). 



Apart from its principal divisions, which, as indicated above, 

 are more or less common to all vertebrates, the external form of the 

 brain in various species is determined by the elaboration of certain 

 parts in comparison with others. In the mammalia the cerebral 

 hemispheres and the cerebellum are the chief form-determinants, 

 although the pons and the corpora quadrigemina also are significant 

 in this connection. It will be seen also that the form of the brain 

 is more or less dependent on the existence at certain places of well- 

 marked flexures (cf. Plate II). The first of these, the cephalic flexure, 

 is in the region of the mesencephalon, the anterior portion of the 

 brain being bent downward; the second, or pontine flexure, a 

 bend in the opposite direction, is at the fourth ventricle; while 

 the thi-rd, or cervical flexure, is at the point where the myelence- 

 phalon passes over into the spinal cord. By these flexures the over- 

 all length of the brain is kept within the dimensions of the cranial 

 cavity. 



The Cranial Nerves 



The peripheral nervous system embraces two groups of paired 

 and, for the most part, metamerically arranged nerves, namely, the 

 spinal nerves — those arising from the spinal cord and leaving the 



mx o 



Fig. 45. Branches of the left ophthalmic nerve In the region of the orbit, 

 dorsal view, after Winckler: f, frontal nerve; 1, lacrimal nerve; mx, maxillary 

 nerve; n, nasociliary nerve; o, orbital nerve. 



