588 Dynamic Theory. 



The Dormouse and Beaver have the " proventriculus, " a little stomach, 

 which, in birds, is at the lower end of the gullet above the gizzard, and 

 is furnished with the gastric glands. The Porcupine and Hedgehog 

 have quills. The Rodentia often have disproportionately developed 

 hind limbs, like the birds. In the Jerboa the three chief metatarsal 

 bones are fused into one, as in the birds. The Bat has wings, and a 

 keel on the breast bone, as have the birds. The Cheiroptera, Insectiv- 

 ora, and some of the Rodents, are apt, like the Reptiles, to fall into a 



FIG. 305. Brain of Agouti, a South American animal 

 resembling a Hare, but larger. 

 a. Medulla oblong-ata. 

 b. Fourth ventricle, 

 c. Middle lobe of Cerebellum. 

 d. Lateral lobes of 



e. Hemispheres of cerebrum (without convolutions). 

 /.Olfactory lobes. ( Owen.) 



state of torpidity, accompanied by the circula- 

 tion of black, or carbonized, blood. The 

 Zizil ( Arctomys citillus ) has a temperature 

 in summer of 103. When torpid, it is re- 

 duced to 80 or 84. The blood of the Mar- 

 mot ( Arctomys Marmota ) is in summer 

 101, and when torpid only 43 R In 

 the Lissencephala the reproductive testes 

 remain in the abdomen or are protruded into 

 a temporary scrotum only at the breeding 

 FIG 305. period to be again retracted. In many of 



them the squamosal and tympanic plates of the skull bones retain their 

 primitive condition of distinct bones. The orbits have not an entire 

 rim of bone. There are two precaval ( vena cavae superior ) veins, as in 

 birds. (See fig. 116.) 



In all the mammals above the marsupials, the corpus callosum is 

 found more or less complete, its degree of development being indicated 

 by its length from front to rear of the cerebrum, and the amount of the 

 inferior deflection of its posterior and anterior extremities called knees. 

 It is formed by the gradual increase of the transverse fibres, which, orig- 

 inally passing over the hippocampal commissure, receive additions to 

 their number at the forward or anterior edge of the lyra, which is the 

 name given to this body of fibres. The effect of this is to push upward 

 and backward the original cross-fibres uniting the hippocampi, until, 

 from a position well in advance, they are crowded back, always forming 

 the rear end of the corpus*callosum. With the increase of the corpus 

 callosum there is a corresponding decrease in the size of the anterior 

 commissure. 



The development of the hippocampus major and the accompanying 

 commissures, the longitudinal fornix and the transverse corpus callosum, 



