DE. J. MUKIE ON THE FORM AJSTD STRUCTURE OF THE MANATEE. 183 



The internal face of the encephalon, in longitudinal meso-vertical section, is shown 

 in fig. 34. In this view the body of the corpus callosum presents well-marked arching 

 from before backwards, being highest in front. It is of considerable depth throughout, 

 thickening very much as it sharply or subangularly bends downwards and backwards in 

 a prominent knee. This latter descends, moreover, by a rostrum of some magnitude, 

 which unites partially with the so-called " precommissural fibres"' of the body of the 

 fornix. The posterior or splenial end of the fornix increases very much in volume, and 

 terminates by a beaked process. At this point the returning commissural or " psalterial 

 fibres " of Flower^, which are shallow as compared with the splenium, pass horizontally 

 forwards and constitute the fornix. This latter augments in thickness and sweeps gently 

 down as its precommissural fibres trend towards and unite with the rostrum and genu 

 of the corpus callosum. Below then- angle of junction is a circular connecting band, 

 the anterior commissure. This is of fair size, in this respect unlike that of the Porpoise 

 (Phoccena), where Professor Huxley avers it is no more than ^ inch in diameter. 



I regret, for many reasons, I have been unable to work out thoroughly the cerebral 

 convolutions of Manatus, as it would have formed a standard of comparison of the 

 Sirenian Order with the other Mammalia. At the same time my faulty material has 

 served as a sketch map sufiiciently complete to recognize the type of folds. Whilst the 

 numerous secondary fissures and folds were in great part destroyed by the firm adhesion 

 of the pia mater, subsequent coalescence of the neighbouring walls, and scaling of the 

 cerebral substance superficially, yet I have been successful in elucidating the main sulci 

 and gyri. Thus, with other points, I offer more than a mere passing glimpse of the 

 structural organization of the brain of this remarkable group, and supply a desideratum 

 craved for by all biologists whose attention has been directed to their internal soft 

 structures. 



As to fissures the main and most striking feature of each hemisphere is the great deep 

 Sylvian cleft (s^). On whichever face the brain is examined it is a conspicuous land- 

 mark, and cuts, as mentioned, each cerebral half into two well-defined areas, anterior 

 and posterior. Commencing at the middle lobe (island of Eeil), on the brain's base, the 

 Svlvian fissure sweeps round to the outer face and ascends almost perpendicularly half- 

 way up, then splits into an anterior and posterior division. For other fissures and gyri 

 consult the figures in PI. XXV. and their descriptions. I may remark of the brain as 

 a whole, that, in shape and type of gyri &c., it appears to follow more that of the 

 Elephant than those of Cetacea generally. Compared with the figure of the Dugong's 

 brain, it is shorter, higher, and proportionally broader. 



The pneumo-gastric nerve issues from the foramen lacerum, at its posterior part, close 

 to the rectus anticus minor. An inch below it sends upwards a laryngeal branch ; and 



' Hiixley, Hunterian Lectures, 1864. 



- " Commissures of the Central Hemispheres of the Marsupialia and Monotremata," PhU. Trans. 1865, 

 p. 634. 



VOL. VIII. — PAET III. September, 1872. 2 b 



