xxv] 



SKULL 



639 



glands. In the lower 

 Mammals these glands 

 arise in connection with 

 rudimentary hairs (the 

 milk hairs) which are 

 later shed. 



As regards their in- 

 ternal structure the 

 great differences be- 

 tween Mammals on the 

 one hand and Reptiles 

 and Birds on the other, 

 are to be found in the 

 skull, the brain and the 

 limbs and, to a lesser 

 extent, in the heart and 

 the arrangement of the 

 great arteries and veins. 



Turning first to the 

 skull, we find that in a 

 Mammal instead of hav- 

 ing only one knob or 

 condyle to fit into a 

 cup on the first vertebra, 

 as is the case with Birds 

 and Reptiles, the skull 

 has two, which are pro- 

 jections of the exocci- 

 pital bones that wall in 

 the sides of the foramen 

 magnum, whereas in 

 Birds the single condyle 

 is an outgrowth of the 

 basi-occipital bone that 

 forms the floor of the 

 foramen magnum (Fig. 

 315). In Reptiles, more 

 especially the Chelonia, 

 the so-called single con- 

 dyle is really trifid, 

 the lateral parts being 



-22 



FIG. 315. Ventral view of the cranium of a Dog, 

 Canis familiar is x f . 



1. Supra-occipital. 2. Foramen magnum. 

 3. Occipital condyle. 4. Tympanic bulla. 

 5. Basi-occipital. 6. Basisphenoid. 7. Ex- 

 ternal auditory meatus. 8. Glenoid fossa. 



9. Foramen lacerum medium, aperture through 

 which the internal carotid passes to the brain. 



10. Postglenoid foramen. 11. Alisphenoid. 

 12. Presphenoid. 13. Vomer. 14. Jugal. 

 15. Pterygoid. 16. Palatal process of 

 palatine. 17. Palatal process of maxilla. 

 18. Posterior palatine foramen. 19. An- 

 terior palatine foramen. 20. Palatal process 

 of premaxilla. 21. Opening of tube in 

 alisphenoid bone through which the carotid 

 artery passes. 22. Hole for passage of 

 Eustachian tube. 23. Process of squamosal 

 to act as a stay for condyle of lower jaw. 



ii xn. Exits of cranial nerves. i 2. Second 

 incisors. c. Canine. pml, pm4. First 

 and fourth premolar. m 1. First molar. 



