24 Field Museum of Natural History — Zoology, Vol. XL 



membranes of the uterus, and through this placenta the embryo is 

 nourished. 



In all female mammals, except the Monotremes, the intestinal 

 and genital openings are separate, but in the latter they open into 

 the cloaca, as in birds. 

 Mammary Glands — These glands secrete milk by which the young are 

 nourished and are present in both sexes in all mammals, but are 

 usually only functional in the female. 



In all except the Monotremes their orifices are situated upon the 

 end of conical elevations called mammilce, or teats, which are taken 

 into the mouth of the young animal. In the Whales the glands are 

 unusually developed and a quantity of milk is injected into the 

 mouth of the young by muscular contraction. In the Monotremes 

 the teats are lacking, the ducts of the mammary glands opening 

 through pore-like orifices in the skin. In addition to a number of 

 other peculiar characters, these strange mammals are claimed to be 

 oviparous, the eggs resembling in development those of a reptile. 



NERVOUS SYSTEM AND ORGANS OF SENSE. 



The Brain — The brain is contained in the cavity of the skull. The 

 greater portion is called the cerebrum; a much smaller portion at the 

 back of the skull cavity is known as the cerebellum; and the com- 

 mencement of the spinal marrow, as the medulla oblongata. The 

 brain of a mammal differs from that of other vertebrates in having 

 the two hemispheres of the cerebellum united by a commissure {pons 

 varolii) and the cerebral hemispheres more or less connected by an 

 anterior and a superior transverse commissure, the corpus c olio sum 

 of anatomical text books. In most mammals the brain is more or 

 less convoluted, highly so in Man, varying in degree in many, or not 

 at all as in some of the Marmosets (Hapale). 



Spinal Cord — The main nerve axis of the body passing through the 

 vertebrae from head to tail, but which it is unnecessary to discuss 

 here. 



The Sense of Touch — An increased supply of nerves and blood-vessels 

 to a part of the skin renders it more sensitive and susceptible to 

 what is called the sense of touch. Depnal susceptibility for this 

 reason differs in various parts of the body. For example, in Man 

 the ends of the fingers, from being supplied abundantly with pencil- 

 late plexuses of nerves, are much more sensitive than portions of the 

 arm or back. The lips and tongue are still more so, the latter 

 usually being capable of distinguishing distinct sensations when 

 touched by the points of a pair of dividers when separated only ./r, 



