GENERAL CHARACTERS OF MAMMALS. 671 



is in front of the acetabulum. The ventral symphysis is 

 usually restricted to the pubes, but in some Insectivores and 

 Bats these do not meet. Except in Echidna, the acetabulum 

 is completely ossified, and there is often a special acetabular 

 bone. The ankle joint is cruro-tarsal. 



The cerebral hemispheres have usually a convoluted surface, 

 and always cover the optic thai a mi and the optic lobes (now 

 four-fold corpora quadrigemina], and in higher forms the 

 cerebellum as well. The commissural system is well developed, 

 being especially represented by a large corpus callosum, except 

 in Monotremes and Marsupials, in which the anterior com- 

 missure is large and the corpus callosum absent or very small. 

 There is also an important set of longitudinal fibres called the 

 fornix. 



Except in Monotremes, in which there is a cloaca, the food 

 canal ends separately from the uroge?iital aperture. 



The heart is four-chambered, and the temperature of the 

 blood is high, though lower than that of Birds. There is but 

 one aortic trunk, which curves over the left bronchus. The 

 red blood corpuscles are, when fully formed, non-nucleated, 

 and are circular in outline, except in the Camelidce, where they 

 are elliptical. There is no renal-portal system. 



The lungs are invested b\ pleura! sacs, and lie freely in 

 the chest cavity. Within the lungs the bronchial tubes fork 

 repeatedly into finer and finer branches. At the top of the 

 trachea there is a complex larynx with the vocal chords. 



The kidneys are generally compact and rounded bodies ; 

 the ureters open into the bladder, except in Monotremes, in 

 which they enter a urogenital sinus. Except in Monotremes, 

 the outlet or urethra of the bladder unites in the male with 

 the genital duct, to form a urogenital canal ; in the female, 

 except in Monotremes and a few other cases, the urethra and 

 the genital duct open into a common vestibule. 



In the more primitive Mammals the testes lie in the 

 abdomen ; in the majority they descend permanently (or in a 

 few cases temporarily] into a single or paired scrota I sac, 

 lying, except in Marsupials, behind the penis. 



The ovaries are small. Except in Monotremes, the genital 

 ducts of the female are differentiated into (a) Fallopian tubes, 

 which catch the ova as they burst from the ovaries : (b) a 

 uterine portion in which the young develop : and (c) a vaginal 



