THE SENSE ORGANS. 283 



lutions (gyri). A commissure (corpus callosum) connecting the two 

 hemispheres is well-developed, and rudimentary only in the Apla- 

 centalia. On the other hand the optic lobes, which are known as 

 the corpora quadrigemina, and are the equivalents of the corpora 

 bigemina of the lower forms, are reduced in size, and are in great 

 part or entirely covered by the posterior lobes of the hemispheres. 

 The pituitary body (hypophysis) and the pineal gland are never 

 absent. The cerebellum in the Aplacentalia resembles that of the 

 birds in the disproportionate development of its median lobe. There 

 are, however, numerous intermediate stages between such a cere- 

 bellum and a cerebellum in which the lateral lobes are largely 

 developed. The pons Varolii also is little developed in the lower 

 forms, but in the higher Mammals is increased to a large swelling 

 at the point where the brain is prolonged into the spinal cord. The 

 twelve cranial nerves are completely separated. The spinal cord 

 usually extends only as far as the sacral region, where it ends with 

 a cauda equina; there is no posterior rhomboidal sinus. 



Sense organs. The olfactory organ presents, on account of the 

 complication of the ethmoidal labyrinth, a greater development of 

 the olfactory mucous membrane than in any other class. The two 

 nasal cavities, which are separated by the median septum, often 

 communicate with spaces in the neighbouring cranial and facial bones 

 (sinus frontales, sphenoidales, maxillares), and open externally by 

 paired apertures ; in the Cetacea, which have no sense of smell, the 

 latter may be fused to form a median opening (Delphinidce). In 

 this case the nasal passages serve only as air-passages. The nasal 

 openings are, as a rule, supported by movable cartilaginous pieces, 

 which in some cases are largely developed and lead to the formation 

 of a proboscis, which is used as a burrowing and tactile organ, and 

 when greatly developed (Elephant) as a prehensile organ. In the 

 diving Mammals the nasal apertures can be closed by muscles 

 (Phocidce) or by valvular apparatuses. A nasal gland is often present 

 on the external wall of the nares, or in the cavity of the upper jaw 

 (maxillary sinus). The olfactory nerve is distributed as in the Birds 

 on the superior turbinal bones, and on the upper parts of the nasal 

 septum. The internal nares are always paired and open into the 

 pharynx, far back at the end of the soft palate. 



The eyes (vol. i., fig. 88) present various degrees of development; 

 they are always small in the Mammals which live beneath the earth, 

 and in some cases (Spalax, Chrysochloris) are quite hidden beneath 

 the skin, and are incapable of receiving luminous impressions. They 



