THE MAMMALIA 297 



quite diagnostic of mammals, for there is no true renal-portal 

 system in birds, but in this class the inferior vena cava 

 originates in front of the kidneys, and not behind them, as 

 in mammals, and there is a pair of so-called renal portal 

 veins which traverse the substance of the kidneys without 

 breaking up into capillaries within it.) 



In connection with the circulatory system, the non-nucleated 

 red corpuscles of mammals should be noticed. They are 

 highly characteristic, and may easily be studied in a drop 

 of human blood drawn from the finger. They are circular 

 bi-concave discs, with swollen margins, and consist of a minimal 

 quantity of a protoplasmic network, the stroma, holding a 

 relatively very large quantity of haemoglobin in its meshes. 

 Human red corpuscles measure 7/x in diameter ; those of the 

 elephant, 9/x, of the dog and rat, 7//., of the cat,v 6p, of the 

 sheep, 5/A (f^ = ^-^f of a millimetre). 



Before passing on to the urogenital system and the develop- 

 ment of the embryo, we may briefly consider the more 

 characteristic features of the central nervous system of 

 mammals. The spinal cord does not require special mention, 

 but the brain is well developed, and remarkable for the great 

 relative size of the cerebral hemispheres, which in the rabbit 

 overlap the thalamencephalon and mid-brain behind it, and 

 in higher forms show a progressive increase in size, till in 

 man they cover and conceal the cerebellum and medulla 

 oblongata. With the increasing size of the hemispheres, we 

 find an increasing complexity due to infoldings of a more or 

 less complicated character, which are expressed by a number 

 of tortuous grooves on their surfaces. These grooves are called 

 sulci, and the raised areas between them are known as gyri. 

 In a low type of placental mammal, such as the rabbit, the 

 sulci are few in number, and shallow. In a higher form, 

 such as the dog, they are much more numerous, and in man 

 their complexity is remarkable. It is further characteristic 

 of the mammalian brain, that the cerebral hemispheres are 

 united by a broad transverse band of commissural nerve fibres 

 called the corpus callosum, and that the dorsal surface of 

 the mid-brain is produced into two pairs of swellings, or 

 corpora quadrigemina, instead of one pair, or corpora bigemina, 

 as in lower vertebrates. 



In the organs of special sense, the most remarkable feature 



