66 MORPHOLOGY OF THE VERTEBRATE BODY 



the nervous system but originates from the roof of the mouth. 

 The infundibulum and the hypophysis together form the pituitary 

 body, an endocrine gland which pours its secretion into the blood. 

 The ventral part of the brain below the optic lobes is known as the 

 crura cerebri. As on the dorsal side, the most posterior region is 

 the medulla oblongata. On the dorsal surface of the medulla 

 and just posterior to the cerebellum, is a mass of blood ves- 

 sels, the posterior choroid plexus ; and on the dorsal side of the 

 diencephalon, a similar mass, the anterior choroid plexus. The 

 spinal cord exhibits a brachial and a lumbar enlargement, in correla- 

 tion with the larger nerves of these regions, and tapers posteriorly 

 as the terminal filament, or filum terminale (Fig. 37). 



Internally, the brain and cord contain a continuous cavity 

 (Fig. 38 B), which is expanded as the ventricles of the brain and 

 reduced to a microscopic canal in the cord. The choroid plexuses, 

 which are located at places where the roof of the neural tube is 

 very thin, are masses of small blood vessels included in the roof 

 of the third and fourth ventricles and projecting downward into 

 these cavities. 



In the frog there are ten pairs of cranial nerves arising from the 

 ])rain (Fig. 39 B), all of them passing to the head and neck region, 

 with the single exception of the tenth pair, or vagus nerves, which 

 have branches running to the lungs, heart, and digestive tract. 

 These ten pairs of cranial nerves occur with slight modifications in 

 all vertebrate animals. In addition, an eleventh and twelfth pair 

 are differentiated as cranial nerves in reptiles, birds, and mam- 

 mals. 



The spinal nerves of vertebrates vary in number according to 

 the length and development of the spinal cord and its protective 

 spinal column. In general, one expects to find a pair of spinal 

 nerves for each well-developed vertebra in the body region. In 

 the human body, there are some thirty pairs of spinal nerves, if we 

 include the rudimentary nerves at the posterior end of the cord. In 

 the adult frog, the vertebrae are reduced to nine and there are nine 

 well-developed spinal nerves and a smaller tenth nerve which is 

 readily seen in dissection (Fig. 37). In the frog tadpole, however, 

 there are rudimentary structures, which correspond to the vertebrae 

 and spinal cord of this posterior region in other animals ; but, as the 

 frog develops, these become a single terminal bone, the urostyle 

 and the terminal filament of the spinal cord (Figs. 37 and 40 B). 



