Chap. III.] GENERAL ANATOMY. 53 



In the spinal cord there are two enlargements; a brachial 

 enlargement between the shoulders, and a lumbar enlargement 

 between the thighs. In the latter is a rhomboidal depression 

 (the sinus rhomboidalis). 



6. Attention may now be drawn to one or two salient points : 



(1) The lungs are closely attached to the body-walls. They 

 are not distensible, and only their ventral surface is covered 

 by a pleural membrane. They communicate with distensible 

 air-sacs. 



(2) The heart is large, and has two ventricles and two auricles. 

 There is no separate sinus venosus or truncus arteriosus. 



(3) There is one aortic arch, the right. 



(4) There is no definite renal-portal system, but the hypo- 

 gastric veins pass through the kidney, and perhaps give off to 

 that organ a few branches. Posteriorly the transverse branch 

 connecting the hypogastrics is in connection with the portal. 



(5) There are separate ducts for the kidneys and the testes in 

 the male. 



(6) There is a cloaca, in the dorsal walls of which are the aper- 

 tures of the urino-gemtal ducts. 



(7) The right ovary and right oviduct atrophy in the adult. 



(8) There is neither urinary bladder nor gall bladder. 



5. General Anatomical Conclusions. Notwithstanding well- 

 marked points of difference, our three air-breathing types, the 

 mammal, the bird, and the amphibian, agree in possessing many 

 fundamental points of resemblance. 



1. All possess a skull and vertebrated back-bone. 



2. The central nervous system is dorsal in position, and pro- 

 tected by the skull and arches of the vertebrae. 



3. The brain is in each case divisible into fore-, mid-, and hind- 

 brain, and the spinal cord is in each case similar in essential 

 form and structure. 



4. All have a visceral nervous system, the sympathetic. 



5. As we shall see hereafter, there is a fundamental similarity 

 in the distribution of the cranial nerves; and in all the heart, the 

 lungs, and the stomach are largely innervated by a cranial nerve. 



