At first the two hemispheres are connected only with their respective thalami. But during the fourth and fifth months of embryonic life they become connected with each other above the thalami through the growth between them of a massive system of transverse fibres which crosses the median line like a great bridge and is called the _corpus callosum_. These fibres radiate in the walls of both hemispheres and form a direct connection between the convolutions of the right and of the left side. Beneath the corpus callosum another system of fibres called the _fornix_ is formed, between which and the corpus callosum there is a peculiar connection. Just in front of the thalami, where the hemispheres begin their growth, a ganglionic mass called the _corpus striatum_ (_C.S._, Figs. 32 and 33) is formed in their wall. It is complex in structure, consisting of two main parts, called _nucleus lenticularis_ and _nucleus candatus_ respectively. The figures, with their respective explanations, will give a better idea of the farther details of structure than any verbal description; so, after some practical directions for dissecting the organ, I will pass to a brief account of the physiological relations of its different parts to each other. =Dissection of Sheep's Brain.=--The way really to understand the brain is to dissect it. The brains of mammals differ only in their proportions, and from the sheep's one can learn all that is essential in man's. The student is therefore strongly urged to dissect a sheep's brain. Full directions of the order of procedure are given in the human dissecting books, e.g. Holden's Practical Anatomy (Churchill), Morrell's Student's Manual of Comparative Anatomy and Guide to Dissection (Longmans), and Foster and Langley's Practical Physiology (Macmillan). For the use of classes who cannot procure these books I subjoin a few practical notes. The instruments needed are a small saw, a chisel with a shoulder, and a hammer with a hook on its handle, all three of which form part of the regular medical autopsy-kit and can be had of surgical-instrument-makers. In addition a scalpel, a pair of scissors, a pair of dissecting-forceps, and a silver probe are required. The solitary student can find home-made substitutes for all these things but the forceps, which he ought to buy.