COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 



297 



the forebrain or prosencephalon, the midbrain or mesencephalon, and the hindbrain or rhomb- 

 encephalon. Subsequently the first and third vesicles subdivide into two. There thus 

 arise the five principal lobes of the brain, arranged in a longitudinal series (Fig. 67). These 

 are named, beginning anteriorly: the telencephalon, the diencephalon or thalamencephalon, 

 the mesencephalon or midbrain, the melencephalon or cerebellum, and the myelencephalon or 

 medulla oblongata. Each of these five divisions becomes further complicated by additional 

 evaginations, foldings, thickenings, etc., but nevertheless remains as a clearly marked region 

 of the adult brain. The parts derived from each of these divisions will be studied in connec- 

 tion with the specimens. The cavity of the brain becomes the ventricles of the adult brain. 

 The spinal cord develops chiefly by thickening of the lateral walls of the original neural 

 tube. The central cavity is reduced to a small canal, the central canal. The cord consists 

 of an internal zone of gray matter and external zone of white matter formed as described above. 



B 



FIG. 67. Diagrams to show the development of the vertebrate brain. A, early stage, showing 

 the three primary brain vesicles, prosencephalon a, mesencephalon b, and rhombencephalon c. B, later 

 stage, showing division of the prosencephalon into telencephalon d and diencephalon e and of the 

 rhombencephalon into metencephalon / and myelencephalon g. C, adult stage of a lower vertebrate, 

 showing enlargement of the telencephalon to form the cerebral hemisphere i, differentiation of the 

 olfactory lobe h, and development of outgrowths j, k, I from the roof of the telencephalon and dien- 

 cephalon and of the infundibulum n from the floor of the diencephalon. D, adult stage of a higher 

 vertebrate, illustrating further enlargement of the cerebral hemisphere i. a, prosencephalon; b, mesen- 

 cephalon; c, rhombencephalon; d, telencephalon; e, diencephalon; /, metencephalon; g, myelen- 

 cephalon or medulla oblongata; h, olfactory lobe or bulb; i, cerebral hemisphere; j t paraphysis or 

 evagination of roof of telencephalon; k, parietal eye and /, pineal body, evaginations from the roof 

 of the diencephalon; m, cerebellum; n, infundibulum; o, pituitary body. (From Parker and Haswell's 

 Textbook of Zoology, courtesy of the Macmillan Company.) 



The dorsal portions of the gray matter are named the dorsal columns, the ventral portions 

 the ventral columns, and in some regions of the cord there are distinct lateral columns (Fig. 68). 

 3. The functional composition of the nervous system. The nervous functions are divisible 

 into two great classes, sensory and motor. The term sensory applies to those impulses 

 which come into the nervous system from the sense organs, to the nerves which conduct 

 such impulses, and to the tracts, nuclei, or areas of the central nervous system which are 

 concerned with these impulses. The dorsal half of the central nervous system is sensory. 

 The term motor is similarly applied to the nerves, tracts, areas, etc., which are concerned 

 with the initiation and conduction from the central nervous system of impulses which excite 

 parts such as the muscles and glands to activity. The ventral half of the central nervous 

 system is motor. The sensory and motor functions are each subdivisible into somatic and 



