A comparative and quantitative anatomical study of the cerebral ventricular system of the dolphin, Tursiops truncatus, and six other mammalian species, including man, has been made in the course of extensive neuroanatomical investigations of the dolphin. Several techniques have been used in these studies to confirm the morphological arrangement and contours of the ventricles. Casts of the ventricles were produced by use of a variety of casting media and methods and their relative merits were investigated and described. Following injection of the brain with casting media, the brain was either totally removed by dissection or digestion or one hemisphere dissected away to reveal the relationships of the ventricular cast to the opposite cerebral hemisphere. Serial sections of the brains of three dolphins were cut in three cardinal planes, and from these a model of the entire ventricular system was reconstructed. Serial sections (Nissl and Loyez) were also used to identify structures closely related to the ventricular cavities. Our results show that components of the cerebral ventricular system of the dolphin reflect, to a considerable degree, many of the specialized features of that brain including brachycephaly, widened and foreshortened temporal lobes, large limbic lobe, hypodeveloped frontal region, absence of a n occipital pole, and massive development of the tectal acoustic apparatus. The morphological characteristics of the ventricular system and the relationship of this system to brain areas is discussed with respect to possible functional and phylogenetic implications.The ventricles of the brain have interested investigators from at least the time of Leonard0 da Vinci, who made the first known ventricular cast (McMurrich, '30). Historical accounts of various aspects of subsequent research on the cerebral ventricular system have been reviewed by Millen and Woollam ('62), Clarke ('62), and Adelmann ('66). The cerebral ventricles are of particular interest for the reason that they are primordial hollows of the brain (as distinct from secondary hollows such as the subarachnoid space, subdural space, cavum septi pellucidi, etc.) and reflect in changes of their configurations and size the development and growth of the neural tube. 1 Present address: New York Aquarium, New York, New York. 1This research was supported by grant NB 03097, National Institute of Neurological Diseases and Blindness, National Institutes of Health, and grants GB 4407 and 5230, National Science Foundation.2The cavum septi pellucidi, as reviewed in detail by Thompson ('32), is a space bounded dorsally by the corpus callosum, posteriorly and ventrally by the fornix, and laterally by the stretched laminae of the septi pellucidi. This space has been called the "fifth" ventricle by Forbes (1882), Elliot Smifh (1896, '00-'03), and later workers. In the dolphin, the cavum septi pellucidi is open anteriorly as it is in the dog, cat and monkey. The "sixth" ventricle or cavum Vergae was reviewed by Dand ('31) as a space bounded laterally bp th...
