Integrative Systems 151 



contain small cavities connecting with the median mesocoele (Fig. 

 136), which is usually very narrow (the "aqueduct of Sylvius" of 

 human anatomy; Fig. 140, a). 



The metencephalon, the anterior division of the embryonic hind- 

 brain, produces a dorsal lobe, the cerebellum, which in most verte- 

 brates attains a bulk that makes it one of the most conspicuous fea- 

 tures of the brain (Figs. 136, 137, 140). The exceptions are the small 

 group of lungfishes and the amphibians, in which the cerebellum is 

 relatively small. The cerebellum is a single lobe of compact form except 

 in birds and mammals, in which it is divided into three regions or 

 lobes, a median vermis and a pair of lateral lobes (Figs. 416, 521). 

 Along with this tripartite lobing, there is developed in the thick ven- 

 tral wall of the metencephalon a massive transverse bridge of nerve- 

 fibers, the pons Varolii (Figs. 139, 140, 530), connecting the two 

 lateral cerebellar lobes. 



The chief function of the cerebellum is the coordination of muscular 

 activity. For example, when a four-legged animal runs, numerous mus- 

 cles of each of the four legs are in action. For smooth operation of the 

 locomotor mechanism as a whole, it is necessary that the contraction of 

 each muscle be precisely timed as to its beginning and duration and 

 that the degree of its contraction be determined. All of this control is 

 effected automatically — i.e., not "voluntarily" — by the nervous 

 mechanism in the cerebellum. 



The myelencephalon produces no prominent lobes and merges, 

 without definite demarcation, into the spinal cord (Figs. 136, 137). Its 

 nervous material is confined to the thick ventral and lateral walls. Its 

 broad roof is entirely a thin non-nervous membrane, most of which is 

 highly vascular and is elaborately infolded to form a choroid plexus, 

 the posterior tela chorioidea (Figs. 137, 138A). The main mass of the 

 lateral and ventral walls consists of longitudinal tracts of fibers which 

 continue forward from the spinal cord into the brain. Certain of these 

 tracts occasion a more or less prominent pair of elongated swellings, 

 the pyramids (Fig. 530), on the ventral surface of the medulla. The 

 tracts of the pyramids continue forward as the crura cerebri of the 

 mesencephalon and thence eventually into the cerebral hemispheres. 

 The broad, shallow cavity of the medulla is the "fourth ventricle." 



The medulla contains the various sensory centers whose receptors 

 are in the skin, the centers for the sense of taste whose receptors are 

 located within the mouth of all vertebrates and also in the external 

 skin of many fishes, and the primary centers for the mechanism of the 

 ears and for various interoceptors of the digestive and respiratory or- 



