BRAIN. SENSE-ORGANS. 495 



They reach their highest development in the Cetacea, the brain 

 of which is even more complexly convoluted than that of mail. 

 A broad commissure, the corpus callosum, connecting the 

 two hemispheres is always present except in Monotremata 

 and Marsupialia. When this is absent the anterior commis- 

 sure is large, and there is an important commissure taking the 

 place of the body of the fornix and called the hippocampal 

 commissure. The latter is placed just above the anterior com- 

 missure. The lateral ventricles possess anterior and descending 

 cornua, and in the Cetacea and Primates a posterior cornu as 

 well. 



The optic lobes, which are four in number and known as the 

 corpora quadrigemina, are reduced in size and are in great part 

 or entirely covered by the posterior lobes of the hemispheres 

 (Fig. 263). The pituitary body (hypophysis) and the pineal body 

 (epiphysis) are always present. There is no parietal organ. 

 The cerebellum consists of a median lobe or vermis and of two 

 lateral lobes each of which carries a small accessory lobe, the 

 flocculus. A transverse commissure, the pons varolii, lying on 

 the ventral surface of the anterior part of the medulla bblongata 

 and connecting the lateral lobes of the cerebellum, is always 

 present, but varies in its development in the different forms. 

 There are twelve pairs of cranial nerves as in Sauropsida. 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 sense of touch is most acute in the skin of 

 the face, lips, tongue and ends of the extremities, the skin in 

 these regions being provided with special organs in which the 

 nerves terminate called tactile corpuscles. The vibrissae or long 

 bristle-like tactile hairs which are often found on the face, and 

 the papillae of which are richly innervated, are special organs of 

 touch. In the same category may be placed the wings and the 

 cutaneous expansions on the faces of bats, which are so sensitive 

 as to enable the animal to detect obstacles without touching 

 them, by the mere alteration in the pressure of the air in their 

 neighbourhood. 



The sense of taste has its seat principally at the root of the 

 tongue (papillae circumvallatae) and on the soft palate. 

 |ii The sense of smell appears to be present in all Mammals 



