NOTES. 387 



the anterior. Dr. Wyman contends that the limbs are tegumentary organs, 

 and attached to the vertebral column in the same sense that the teeth are 

 attached to the jaws. Other theories are that they originate from gill-arches 

 (Gegenbaur) or that they are remains of a once continuous lateral fin (Thacher). 



79 The first trace of muscular tissue is found in the stem of Vorticella an 

 Infusorian. In Hydra we find neuro-muscular cells, and the Jelly-fishes have 

 muscular tissue. 



80 The muscles of some Invertebrates, as Spiders, are yellow. 



81 The muscles of the heart and gullet are striped. In the lower animals 

 these distinctions of voluntary and involuntary, striated and smooth, solid and 

 hollow, muscles can seldom be made. 



83 The skeleton of the Carrion-crow, for example, weighs, when dry, only 

 twenty-three grains. 



13 The Dragon-fly can outstrip the Swallow ; nay, it can do in the air more 

 than any bird it can fly backward and sidelong, to right or left, as well as 

 forward, and alter its course on the instant without turning. It makes twen- 

 ty-eight beats per second with its wings, while the Bee makes one hundred 

 and ninety, and the House-fly three hundred and thirty. The swiftest Race- 

 horse can double the rate of the Salmon. So that Insect, Bird, Quadruped, 

 and Fish would be the order according to velocity of movement. 



84 The theory that flies adhere by atmospheric pressure is now abandoned. 

 84 More precisely, the term brain, or brains, applies only to the cerebrum, 



while the total contents of the cranium are called encephalon. 



86 The exact functions of the cerebrum are not yet clearly understood. If 

 we remove it from Fishes, or even Birds, their voluntary movements are little 

 affected, while the Amphioxus, the lowest of Fishes, has no brain at all, but 

 its life is regulated by the spinal cord. Such mutilated animals, however, 

 make no intelligent efforts. The substance of the cerebrum, as also the cere- 

 bellum, is insensible, and may be cut away without pain to the animal ; and 

 when both are thus removed, the animal still retains sensation, but not con- 

 sciousness. 



87 It is very difficult to define sensation, or sensibility. The power is pos- 

 sessed by animals which have neither nervous system nor consciousness. 

 These low manifestations of sensibility are called irritability the power by 

 which an animal is capable of definitely responding to a stimulus from with- 

 out. The response is not called out by the direct action of the stimulus, but 

 is determined mainly by the internal structure and condition of the animal. 



88 Parts destitute of blood-vessels, as hair, teeth, nails, cartilage, etc., are 

 not sensitive. The impressibility of the nerves is proportioned to the activity 

 of circulation. According to the recent investigations of Dr. Bowditch, the 

 channels of motor and sensitive impressions lie in the lateral, and not in the 

 anterior and posterior columns of the spinal cord. 



i9 " Tentacles " and " horns " are more or less retractile, while antennae are 

 not, but all are hollow. Antennae alone are jointed. 



In Man, the soft palate and tonsils also have the power of tasting. 



" No organ of hearing has been discovered with certainty in the Radiates 

 and Spiders. The " ear " of many lower animals is probably an organ for 

 perceiving the animal's position rather than sound an "equilibrium organ." 



