584 



CETACEA. 



anterior part of the vermiform process of the 

 cerebellum. 



The pineal gland is a small flattened body 

 about two lines in length, connected as usual 

 to the thalami optici. These appear in each 

 ventricle in the form of an oval flattened 

 body (i, Jig. 273). They are joined together 

 posteriorly by the medullary commissure. 

 Tiedemann did not observe any soft commis- 

 sure. 



The third ventricle is continued anteriorly 

 into the infundibulum. 



The corpora striata (d) are proportionally of 

 small size, as Hunter observed in the brain 

 of the Whale. They are united anteriorly by 

 the anterior commissure. 



The fornix is also of inconsiderable size. 

 The slender anterior pillars of the fornix proceed 

 to the mammillary bodies, and send forwards 

 two small triangular medullary lamellae to the 

 under surface of the anterior part of the corpus 

 striatum, from which the septum lucidum is 

 continued. The fornix then bends backwards 

 along the under surface of the corpus callosum 

 and above the thalami, and its hinder crura 

 sink down, diverging from each other to form 

 the cornua ammonis (g). These bodies are 

 small, thin, but broad, and exhibited no den- 

 ticulated folds. The taenia nmbriata (h) are 

 attached as usual to the external border of the 

 cornua. 



The lateral ventricles are capacious though 

 short; they extend, as in the human brain, 

 into an anterior, a middle, and a posterior horn ; 

 the latter, however, is very small. In each 

 ventricle there is a large plexus choroides, which 

 is remarkable for the transverse parallel folds 

 of membrane which support the divisions of 

 the artery. 



With respect to the cerebral nerves, Tiede- 

 mann states that, although in the Dolphin the 

 brain was removed with every precaution from 

 the skull, yet he could not perceive the slightest 

 trace of the olfactory pair. Hunter and Tyson 

 equally failed to detect them in the Porpesse. 

 Treviranus, however, believed that with the 

 aid of a magnifying glass he had detected very 

 delicate filaments in the situation of the olfac- 

 tory nerves in the Porpesse. But supposing 

 that there was no illusion here, which could 

 hardly have happened to so accurate and close 

 an observer, these fibres represent only a very 

 rudimental condition of the olfactory nerves; 

 and we may observe that the shortness of the 

 anterior lobes of the brain, and the smallness 

 of the striated bodies are closely related to the 

 absence or imperfect development of the first 

 pair of nerves. 



With respect to the other cerebral nerves, 

 they are relatively larger in proportion to the 

 brain than in man. The optic nerves (2, .fig. 

 272) rise partly from the thalami, partly from 

 the anterior bigeminal bodies and the corpora 

 geniculata; they curve round the crura cerebri, 

 and unite as usual before the pituitary gland. 

 The angle at which the nerves diverge from 

 each other after the decussalion is more open 

 than in other Mammalia. 



The accessory nerves of the eye are of large 



size, as the third (3), th fourth (4), and the 

 sixth (6) pair. 



The fifth pair (5), which emerge from the 

 sides of the pons, but arise from the medulla 

 oblongata between the corpora restiformia and 

 olivaria, have a smaller proportional size than 

 in man. 



The nerves concerned in the actions of 

 respiration, as the facial (7), the pneumogastric 

 (10), and the recurrent (11), are well deve- 

 loped, in relation to the large size of the 

 muscles which effect the respiratory movements 

 in the dense medium of water. 



The glosso-pharyngeal nerve (9) and the 

 lingual (12) are also very large, corresponding 

 to the vigorous associated actions of the tongue 

 and pharynx, which must take place during 

 deglutition in the Cetacea. 



But perhaps the most remarkable nerve for 

 its great relative size is the acoustic (8), which 

 certainly testifies to the delicate sense of hear- 

 ing in the Dolphins.] 



The organs of the senses, with the exception 

 of that of smell, are composed, in all the 

 Cetaceans, of the parts which essentially con- 

 stitute them in terrestrial Mammalia, and are 

 only modified with reference to the habitually 

 aquatic life of the animals of this order. But 

 little inquiry has been made as to their utility 

 in these animals, the length of time they con- 

 tinue serviceable, and the characteristic diffe- 

 rences which might be drawn from them for 

 the distinction of the species. 



Eye. The eye of the Herbivorous Cetaceans 

 alone is provided with a lateral lid or membrana 

 nictitans; that of the Spouting Whales is de- 

 void of lachrymal glands; but its lids are fur- 

 nished below with little glands which secrete a 

 mucous matter, adapted like the tears for 

 lubricating the sclerotica. 



[Hunter observes that " the eye in this 

 tribe of animals is constructed upon nearly 

 the same principle as that of quadrupeds, dif- 

 fering, however, in some circumstances ; by 

 which it is probably better adapted to see in 

 the medium through which the light is to pass. 

 It is upon the whole small for the size of the 

 animal, which would lead to the supposition 

 that their locomotion is not great; for, I believe, 

 animals that swim are in this respect similar to 

 those that fly ; and as this tribe come to the 

 surface of the medium in which they live, they 

 may be considered in the same view witli birds 

 which soar ; and we find, birds that fly to 

 great heights, and move through a considerable 

 space, in search of food, have their eyes larger 

 in proportion to their size. 



" The eyelids have but little motion, and 

 do not consist of loose cellular membrane, as 

 in quadrupeds, but rather of the common 

 adipose membrane of the body ; the connexion, 

 however, of their circumference with the com- 

 mon integuments is loose, the cellular mem- 

 brane being less loaded with oil, which allows 

 of a slight fold being made upon the sur- 

 rounding parts in opening the eyelids. This 

 is not to an equal degree in them all, being 

 less so in the Porpoise than in the Piked 

 Whale. 



