12 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



internal secretion as well as a lymphatic organ, has already been noted. The 

 pseudothyroids are a pair of small round reddish masses, one on each side a little 

 anterior to the heart, at about the level of the posterior end of the body of the 

 hyoid. A thyroid gland is located directly under each pseudothyroid, much 

 deeper down in contact with the hyoid cartilage (see Holmes, Fig. 60, p. 222). 

 It is generally difficult to identify the thyroid glands with certainty. For the 

 location of other ductless glands of the frog and for a discussion of their functions 

 consult Holmes, chapter xii, p. 219. 



7. Nervous system. The nervous system is divisible into three parts: the 

 central nervous system, consisting of the brain and the spinal cord; the peripheral 

 nervous system, consisting of the nerves which pass from the brain (cranial 

 nerves) and from the spinal cord (spinal nerves) to all parts of the body; and 

 the sympathetic system, an outgrowth of the central nervous system, differentiated 

 to control the involuntary activities of the body (as digestive tract, heart, etc.). 



Turn the frog back upward and remove the muscles from the head and 

 median portion of the back. This exposes the skull in the head, and a row of 

 spinelike projections in the median line of the back, which are the neural arches 

 of the vertebrae. Between the posterior end of the skull and the first vertebra 

 is a space. Insert one blade of a fine scissors in this space, keeping the point 

 well up against the bone to avoid punching it into the soft brain underneath, and 

 cut away the roof of the skull. The white-lobed brain lying in a cavity in the 

 skull is thus revealed. Its parts will be studied later. Similarly, cut posteriorly 

 through the arches of the vertebrae, first on one side, then on the other, removing 

 their median portions, piece by piece. The neural arches of the vertebrae are 

 thus seen to inclose a continuous space, the neural canal, in which the spinal cord 

 is situated. 



The cranial nerves cannot be observed at present. The spinal nerves arise 

 in pairs from the spinal cord at regular intervals, one pair emerging between 

 two successive vertebrae. Turn the frog ventral side up and look into the 

 cisterna magna. The stout white cords which appear here closely applied to the 

 muscles of the dorsal body wall are the most posterior spinal nerves. 



The sympathetic nervous system consists mainly of a nervous strand on 

 either side of and ventral to the spinal column. At regular intervals along these 

 strands there occur enlargements, or ganglia, each of which connects by means 

 of a nerve, the ramus communicans, with the adjacent spinal nerve. Have the 

 assistant help you find the sympathetic strands and ganglia in the roof of the 

 cisterna magna, alongside the dorsal aorta. 



8. Sense organs. The principal sense organs, the olfactory sacs, the eyes, 

 and the ears, have already been noted. In addition there are sense organs in the 

 skin, which are sensitive to touch, light, chemicals, and differences of temperature, 

 and taste organs in the lining membrane of the mouth. 



