60 COMPARATIVE ANATOMY. 



Arterial System. On the left side of the heart the three 

 trunks into which the ' bulbus arteriosus' divides on that side 

 are also to be seen. A slip of blue paper has been placed be- 

 neath them : the most anterior of the three is the carotid. The 

 central one, which is the systemic aorta, after giving off the 

 subclavian and vertebral arteries (the roots only of these are 

 seen), passes downwards and joins its fellow of the opposite side, 

 to form the abdominal aorta. The mesenteric artery which sup- 

 plies the intestines is given off immediately below this junction. 

 The third trunk is the pulmo-cutaneous artery the pulmonary 

 branches, proceeding to the lungs, only being seen, as the cuta- 

 neous branch has been removed. After giving off the mesen- 

 teric artery, the abdominal aorta disappears behind the kidney, 

 though at some distance interiorly one of its branches, the 

 crural artery, passes downwards over a black bristle, to supply 

 the right leg. 



94. A Frog, dissected to shew its nervous system. 



The nervous system consists of a brain and spinal cord. The 

 cerebral hemispheres are much, elongated, and are, as in all Ver- 

 tebrata except fishes, the largest division of the encephalon. 

 Olfactory nerves are given off from their anterior extremities to 

 the nasal sacs. The optic thalami which succeed are largely 

 exposed between the hemispheres and the corpora bigemina. 

 The latter are comparatively small. The fourth ventricle is 

 widely exposed, and the only representative of a cerebellum is 

 the narrow band of transverse fibres which is to be seen crossing 

 over the anterior part of the ventricle. The spinal cord is short 

 and stout, and nerves are given off from it to the number of ten 

 pairs. The pia mater of the cord is continued as a thin sheath, 

 destitute of nervous matter, to the extremity of the neural canal. 

 The last two or three pairs of spinal nerves pass backwards in 

 the spinal canal for some little distance before passing out to 

 their peripheral distribution. The ganglia on the posterior 

 roots at the point where they join the anterior roots are to be 

 seen. A slip of blue paper has been placed under the two 

 ganglia. On the ventral surface of the throat slips of blue 

 paper have been placed beneath two cranial nerves and the first 

 spinal, viz. the glossopharyngeal, pneumogastric and hypoglossal. 



95. A Frog, dissected to shew certain features not shewn, 



